diff --git a/cmake/toml11Config.cmake__ b/cmake/toml11Config.cmake__
new file mode 100644
index 00000000..edc73f69
--- /dev/null
+++ b/cmake/toml11Config.cmake__
@@ -0,0 +1,2 @@
+@PACKAGE_INIT@
+include("@PACKAGE_toml11_install_cmake_dir@/toml11Targets.cmake")
diff --git a/res/icon/melon_192x192.png b/res/icon/melon_192x192.png
new file mode 100644
index 00000000..b8d4da5d
Binary files /dev/null and b/res/icon/melon_192x192.png differ
diff --git a/res/icon/splash.xcf b/res/icon/splash.xcf
new file mode 100644
index 00000000..594ecab0
Binary files /dev/null and b/res/icon/splash.xcf differ
diff --git a/src/frontend/qt_sdl/Config.cpp b/src/frontend/qt_sdl/Config.cpp
index 02be5b65..7d5331fc 100644
--- a/src/frontend/qt_sdl/Config.cpp
+++ b/src/frontend/qt_sdl/Config.cpp
@@ -816,7 +816,7 @@ Table GetLocalTable(int instance)
 
     std::string key = "Instance" + std::to_string(instance);
     toml::value& tbl = RootTable[key];
-    if (tbl.is_uninitialized())
+    if (tbl.is_empty())
         RootTable[key] = RootTable["Instance0"];
 
     return Table(tbl, key);
diff --git a/src/frontend/qt_sdl/Config.h b/src/frontend/qt_sdl/Config.h
index 9e6d3ea4..39278c36 100644
--- a/src/frontend/qt_sdl/Config.h
+++ b/src/frontend/qt_sdl/Config.h
@@ -25,7 +25,7 @@
 #include <unordered_map>
 #include <tuple>
 
-#include "toml/toml/value.hpp"
+#include "toml/toml11/types.hpp"
 
 namespace Config
 {
diff --git a/src/frontend/qt_sdl/toml/toml.hpp b/src/frontend/qt_sdl/toml/toml.hpp
index 6b0ca1ed..75cc95be 100644
--- a/src/frontend/qt_sdl/toml/toml.hpp
+++ b/src/frontend/qt_sdl/toml/toml.hpp
@@ -1,38 +1,62 @@
-/*
- * The MIT License (MIT)
- *
- * Copyright (c) 2017 Toru Niina
- *
- * Permission is hereby granted, free of charge, to any person obtaining a copy
- * of this software and associated documentation files (the "Software"), to deal
- * in the Software without restriction, including without limitation the rights
- * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
- * copies of the Software, and to permit persons to whom the Software is
- * furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
- * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
- * THE SOFTWARE.
- */
+#ifndef TOML11_TOML_HPP
+#define TOML11_TOML_HPP
 
-#ifndef TOML_FOR_MODERN_CPP
-#define TOML_FOR_MODERN_CPP
+// The MIT License (MIT)
+//
+// Copyright (c) 2017-now Toru Niina
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
 
-#define TOML11_VERSION_MAJOR 3
-#define TOML11_VERSION_MINOR 7
-#define TOML11_VERSION_PATCH 1
+// IWYU pragma: begin_exports
+#include "toml11/color.hpp"
+#include "toml11/comments.hpp"
+#include "toml11/compat.hpp"
+#include "toml11/context.hpp"
+#include "toml11/conversion.hpp"
+#include "toml11/datetime.hpp"
+#include "toml11/error_info.hpp"
+#include "toml11/exception.hpp"
+#include "toml11/find.hpp"
+#include "toml11/format.hpp"
+#include "toml11/from.hpp"
+#include "toml11/get.hpp"
+#include "toml11/into.hpp"
+#include "toml11/literal.hpp"
+#include "toml11/location.hpp"
+#include "toml11/ordered_map.hpp"
+#include "toml11/parser.hpp"
+#include "toml11/region.hpp"
+#include "toml11/result.hpp"
+#include "toml11/scanner.hpp"
+#include "toml11/serializer.hpp"
+#include "toml11/skip.hpp"
+#include "toml11/source_location.hpp"
+#include "toml11/spec.hpp"
+#include "toml11/storage.hpp"
+#include "toml11/syntax.hpp"
+#include "toml11/traits.hpp"
+#include "toml11/types.hpp"
+#include "toml11/utility.hpp"
+#include "toml11/value.hpp"
+#include "toml11/value_t.hpp"
+#include "toml11/version.hpp"
+#include "toml11/visit.hpp"
+// IWYU pragma: end_exports
 
-#include "toml/parser.hpp"
-#include "toml/literal.hpp"
-#include "toml/serializer.hpp"
-#include "toml/get.hpp"
-#include "toml/macros.hpp"
-
-#endif// TOML_FOR_MODERN_CPP
+#endif// TOML11_TOML_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/color.hpp b/src/frontend/qt_sdl/toml/toml/color.hpp
deleted file mode 100644
index 4cb572cb..00000000
--- a/src/frontend/qt_sdl/toml/toml/color.hpp
+++ /dev/null
@@ -1,64 +0,0 @@
-#ifndef TOML11_COLOR_HPP
-#define TOML11_COLOR_HPP
-#include <cstdint>
-#include <ostream>
-
-#ifdef TOML11_COLORIZE_ERROR_MESSAGE
-#define TOML11_ERROR_MESSAGE_COLORIZED true
-#else
-#define TOML11_ERROR_MESSAGE_COLORIZED false
-#endif
-
-namespace toml
-{
-
-// put ANSI escape sequence to ostream
-namespace color_ansi
-{
-namespace detail
-{
-inline int colorize_index()
-{
-    static const int index = std::ios_base::xalloc();
-    return index;
-}
-} // detail
-
-inline std::ostream& colorize(std::ostream& os)
-{
-    // by default, it is zero.
-    os.iword(detail::colorize_index()) = 1;
-    return os;
-}
-inline std::ostream& nocolorize(std::ostream& os)
-{
-    os.iword(detail::colorize_index()) = 0;
-    return os;
-}
-inline std::ostream& reset  (std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[00m";} return os;}
-inline std::ostream& bold   (std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[01m";} return os;}
-inline std::ostream& grey   (std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[30m";} return os;}
-inline std::ostream& red    (std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[31m";} return os;}
-inline std::ostream& green  (std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[32m";} return os;}
-inline std::ostream& yellow (std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[33m";} return os;}
-inline std::ostream& blue   (std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[34m";} return os;}
-inline std::ostream& magenta(std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[35m";} return os;}
-inline std::ostream& cyan   (std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[36m";} return os;}
-inline std::ostream& white  (std::ostream& os)
-{if(os.iword(detail::colorize_index()) == 1) {os << "\033[37m";} return os;}
-} // color_ansi
-
-// ANSI escape sequence is the only and default colorization method currently
-namespace color = color_ansi;
-
-} // toml
-#endif// TOML11_COLOR_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/combinator.hpp b/src/frontend/qt_sdl/toml/toml/combinator.hpp
deleted file mode 100644
index 33ecca1e..00000000
--- a/src/frontend/qt_sdl/toml/toml/combinator.hpp
+++ /dev/null
@@ -1,306 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_COMBINATOR_HPP
-#define TOML11_COMBINATOR_HPP
-#include <cassert>
-#include <cctype>
-#include <cstdio>
-
-#include <array>
-#include <iomanip>
-#include <iterator>
-#include <limits>
-#include <type_traits>
-
-#include "region.hpp"
-#include "result.hpp"
-#include "traits.hpp"
-#include "utility.hpp"
-
-// they scans characters and returns region if it matches to the condition.
-// when they fail, it does not change the location.
-// in lexer.hpp, these are used.
-
-namespace toml
-{
-namespace detail
-{
-
-// to output character as an error message.
-inline std::string show_char(const char c)
-{
-    // It suppresses an error that occurs only in Debug mode of MSVC++ on Windows.
-    // I'm not completely sure but they check the value of char to be in the
-    // range [0, 256) and some of the COMPLETELY VALID utf-8 character sometimes
-    // has negative value (if char has sign). So here it re-interprets c as
-    // unsigned char through pointer. In general, converting pointer to a
-    // pointer that has different type cause UB, but `(signed|unsigned)?char`
-    // are one of the exceptions. Converting pointer only to char and std::byte
-    // (c++17) are valid.
-    if(std::isgraph(*reinterpret_cast<unsigned char const*>(std::addressof(c))))
-    {
-        return std::string(1, c);
-    }
-    else
-    {
-        std::array<char, 5> buf;
-        buf.fill('\0');
-        const auto r = std::snprintf(
-                buf.data(), buf.size(), "0x%02x", static_cast<int>(c) & 0xFF);
-        (void) r; // Unused variable warning
-        assert(r == static_cast<int>(buf.size()) - 1);
-        return std::string(buf.data());
-    }
-}
-
-template<char C>
-struct character
-{
-    static constexpr char target = C;
-
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        if(loc.iter() == loc.end()) {return none();}
-        const auto first = loc.iter();
-
-        const char c = *(loc.iter());
-        if(c != target)
-        {
-            return none();
-        }
-        loc.advance(); // update location
-
-        return ok(region(loc, first, loc.iter()));
-    }
-};
-template<char C>
-constexpr char character<C>::target;
-
-// closed interval [Low, Up]. both Low and Up are included.
-template<char Low, char Up>
-struct in_range
-{
-    // assuming ascii part of UTF-8...
-    static_assert(Low <= Up, "lower bound should be less than upper bound.");
-
-    static constexpr char upper = Up;
-    static constexpr char lower = Low;
-
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        if(loc.iter() == loc.end()) {return none();}
-        const auto first = loc.iter();
-
-        const char c = *(loc.iter());
-        if(c < lower || upper < c)
-        {
-            return none();
-        }
-
-        loc.advance();
-        return ok(region(loc, first, loc.iter()));
-    }
-};
-template<char L, char U> constexpr char in_range<L, U>::upper;
-template<char L, char U> constexpr char in_range<L, U>::lower;
-
-// keep iterator if `Combinator` matches. otherwise, increment `iter` by 1 char.
-// for detecting invalid characters, like control sequences in toml string.
-template<typename Combinator>
-struct exclude
-{
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        if(loc.iter() == loc.end()) {return none();}
-        auto first = loc.iter();
-
-        auto rslt = Combinator::invoke(loc);
-        if(rslt.is_ok())
-        {
-            loc.reset(first);
-            return none();
-        }
-        loc.reset(std::next(first)); // XXX maybe loc.advance() is okay but...
-        return ok(region(loc, first, loc.iter()));
-    }
-};
-
-// increment `iter`, if matches. otherwise, just return empty string.
-template<typename Combinator>
-struct maybe
-{
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        const auto rslt = Combinator::invoke(loc);
-        if(rslt.is_ok())
-        {
-            return rslt;
-        }
-        return ok(region(loc));
-    }
-};
-
-template<typename ... Ts>
-struct sequence;
-
-template<typename Head, typename ... Tail>
-struct sequence<Head, Tail...>
-{
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        const auto first = loc.iter();
-        auto rslt = Head::invoke(loc);
-        if(rslt.is_err())
-        {
-            loc.reset(first);
-            return none();
-        }
-        return sequence<Tail...>::invoke(loc, std::move(rslt.unwrap()), first);
-    }
-
-    // called from the above function only, recursively.
-    template<typename Iterator>
-    static result<region, none_t>
-    invoke(location& loc, region reg, Iterator first)
-    {
-        const auto rslt = Head::invoke(loc);
-        if(rslt.is_err())
-        {
-            loc.reset(first);
-            return none();
-        }
-        reg += rslt.unwrap(); // concat regions
-        return sequence<Tail...>::invoke(loc, std::move(reg), first);
-    }
-};
-
-template<typename Head>
-struct sequence<Head>
-{
-    // would be called from sequence<T ...>::invoke only.
-    template<typename Iterator>
-    static result<region, none_t>
-    invoke(location& loc, region reg, Iterator first)
-    {
-        const auto rslt = Head::invoke(loc);
-        if(rslt.is_err())
-        {
-            loc.reset(first);
-            return none();
-        }
-        reg += rslt.unwrap(); // concat regions
-        return ok(reg);
-    }
-};
-
-template<typename ... Ts>
-struct either;
-
-template<typename Head, typename ... Tail>
-struct either<Head, Tail...>
-{
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        const auto rslt = Head::invoke(loc);
-        if(rslt.is_ok()) {return rslt;}
-        return either<Tail...>::invoke(loc);
-    }
-};
-template<typename Head>
-struct either<Head>
-{
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        return Head::invoke(loc);
-    }
-};
-
-template<typename T, typename N>
-struct repeat;
-
-template<std::size_t N> struct exactly{};
-template<std::size_t N> struct at_least{};
-struct unlimited{};
-
-template<typename T, std::size_t N>
-struct repeat<T, exactly<N>>
-{
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        region retval(loc);
-        const auto first = loc.iter();
-        for(std::size_t i=0; i<N; ++i)
-        {
-            auto rslt = T::invoke(loc);
-            if(rslt.is_err())
-            {
-                loc.reset(first);
-                return none();
-            }
-            retval += rslt.unwrap();
-        }
-        return ok(std::move(retval));
-    }
-};
-
-template<typename T, std::size_t N>
-struct repeat<T, at_least<N>>
-{
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        region retval(loc);
-
-        const auto first = loc.iter();
-        for(std::size_t i=0; i<N; ++i)
-        {
-            auto rslt = T::invoke(loc);
-            if(rslt.is_err())
-            {
-                loc.reset(first);
-                return none();
-            }
-            retval += rslt.unwrap();
-        }
-        while(true)
-        {
-            auto rslt = T::invoke(loc);
-            if(rslt.is_err())
-            {
-                return ok(std::move(retval));
-            }
-            retval += rslt.unwrap();
-        }
-    }
-};
-
-template<typename T>
-struct repeat<T, unlimited>
-{
-    static result<region, none_t>
-    invoke(location& loc)
-    {
-        region retval(loc);
-        while(true)
-        {
-            auto rslt = T::invoke(loc);
-            if(rslt.is_err())
-            {
-                return ok(std::move(retval));
-            }
-            retval += rslt.unwrap();
-        }
-    }
-};
-
-} // detail
-} // toml
-#endif// TOML11_COMBINATOR_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/datetime.hpp b/src/frontend/qt_sdl/toml/toml/datetime.hpp
deleted file mode 100644
index 36db1e10..00000000
--- a/src/frontend/qt_sdl/toml/toml/datetime.hpp
+++ /dev/null
@@ -1,631 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_DATETIME_HPP
-#define TOML11_DATETIME_HPP
-#include <cstdint>
-#include <cstdlib>
-#include <ctime>
-
-#include <array>
-#include <chrono>
-#include <iomanip>
-#include <ostream>
-#include <tuple>
-
-namespace toml
-{
-
-// To avoid non-threadsafe std::localtime. In C11 (not C++11!), localtime_s is
-// provided in the absolutely same purpose, but C++11 is actually not compatible
-// with C11. We need to dispatch the function depending on the OS.
-namespace detail
-{
-// TODO: find more sophisticated way to handle this
-#if defined(_MSC_VER)
-inline std::tm localtime_s(const std::time_t* src)
-{
-    std::tm dst;
-    const auto result = ::localtime_s(&dst, src);
-    if (result) { throw std::runtime_error("localtime_s failed."); }
-    return dst;
-}
-inline std::tm gmtime_s(const std::time_t* src)
-{
-    std::tm dst;
-    const auto result = ::gmtime_s(&dst, src);
-    if (result) { throw std::runtime_error("gmtime_s failed."); }
-    return dst;
-}
-#elif (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 1) || defined(_XOPEN_SOURCE) || defined(_BSD_SOURCE) || defined(_SVID_SOURCE) || defined(_POSIX_SOURCE)
-inline std::tm localtime_s(const std::time_t* src)
-{
-    std::tm dst;
-    const auto result = ::localtime_r(src, &dst);
-    if (!result) { throw std::runtime_error("localtime_r failed."); }
-    return dst;
-}
-inline std::tm gmtime_s(const std::time_t* src)
-{
-    std::tm dst;
-    const auto result = ::gmtime_r(src, &dst);
-    if (!result) { throw std::runtime_error("gmtime_r failed."); }
-    return dst;
-}
-#else // fallback. not threadsafe
-inline std::tm localtime_s(const std::time_t* src)
-{
-    const auto result = std::localtime(src);
-    if (!result) { throw std::runtime_error("localtime failed."); }
-    return *result;
-}
-inline std::tm gmtime_s(const std::time_t* src)
-{
-    const auto result = std::gmtime(src);
-    if (!result) { throw std::runtime_error("gmtime failed."); }
-    return *result;
-}
-#endif
-} // detail
-
-enum class month_t : std::uint8_t
-{
-    Jan =  0,
-    Feb =  1,
-    Mar =  2,
-    Apr =  3,
-    May =  4,
-    Jun =  5,
-    Jul =  6,
-    Aug =  7,
-    Sep =  8,
-    Oct =  9,
-    Nov = 10,
-    Dec = 11
-};
-
-struct local_date
-{
-    std::int16_t year;   // A.D. (like, 2018)
-    std::uint8_t month;  // [0, 11]
-    std::uint8_t day;    // [1, 31]
-
-    local_date(int y, month_t m, int d)
-        : year (static_cast<std::int16_t>(y)),
-          month(static_cast<std::uint8_t>(m)),
-          day  (static_cast<std::uint8_t>(d))
-    {}
-
-    explicit local_date(const std::tm& t)
-        : year (static_cast<std::int16_t>(t.tm_year + 1900)),
-          month(static_cast<std::uint8_t>(t.tm_mon)),
-          day  (static_cast<std::uint8_t>(t.tm_mday))
-    {}
-
-    explicit local_date(const std::chrono::system_clock::time_point& tp)
-    {
-        const auto t    = std::chrono::system_clock::to_time_t(tp);
-        const auto time = detail::localtime_s(&t);
-        *this = local_date(time);
-    }
-
-    explicit local_date(const std::time_t t)
-        : local_date(std::chrono::system_clock::from_time_t(t))
-    {}
-
-    operator std::chrono::system_clock::time_point() const
-    {
-        // std::mktime returns date as local time zone. no conversion needed
-        std::tm t;
-        t.tm_sec   = 0;
-        t.tm_min   = 0;
-        t.tm_hour  = 0;
-        t.tm_mday  = static_cast<int>(this->day);
-        t.tm_mon   = static_cast<int>(this->month);
-        t.tm_year  = static_cast<int>(this->year) - 1900;
-        t.tm_wday  = 0; // the value will be ignored
-        t.tm_yday  = 0; // the value will be ignored
-        t.tm_isdst = -1;
-        return std::chrono::system_clock::from_time_t(std::mktime(&t));
-    }
-
-    operator std::time_t() const
-    {
-        return std::chrono::system_clock::to_time_t(
-                std::chrono::system_clock::time_point(*this));
-    }
-
-    local_date() = default;
-    ~local_date() = default;
-    local_date(local_date const&) = default;
-    local_date(local_date&&)      = default;
-    local_date& operator=(local_date const&) = default;
-    local_date& operator=(local_date&&)      = default;
-};
-
-inline bool operator==(const local_date& lhs, const local_date& rhs)
-{
-    return std::make_tuple(lhs.year, lhs.month, lhs.day) ==
-           std::make_tuple(rhs.year, rhs.month, rhs.day);
-}
-inline bool operator!=(const local_date& lhs, const local_date& rhs)
-{
-    return !(lhs == rhs);
-}
-inline bool operator< (const local_date& lhs, const local_date& rhs)
-{
-    return std::make_tuple(lhs.year, lhs.month, lhs.day) <
-           std::make_tuple(rhs.year, rhs.month, rhs.day);
-}
-inline bool operator<=(const local_date& lhs, const local_date& rhs)
-{
-    return (lhs < rhs) || (lhs == rhs);
-}
-inline bool operator> (const local_date& lhs, const local_date& rhs)
-{
-    return !(lhs <= rhs);
-}
-inline bool operator>=(const local_date& lhs, const local_date& rhs)
-{
-    return !(lhs < rhs);
-}
-
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, const local_date& date)
-{
-    os << std::setfill('0') << std::setw(4) << static_cast<int>(date.year )     << '-';
-    os << std::setfill('0') << std::setw(2) << static_cast<int>(date.month) + 1 << '-';
-    os << std::setfill('0') << std::setw(2) << static_cast<int>(date.day  )    ;
-    return os;
-}
-
-struct local_time
-{
-    std::uint8_t  hour;        // [0, 23]
-    std::uint8_t  minute;      // [0, 59]
-    std::uint8_t  second;      // [0, 60]
-    std::uint16_t millisecond; // [0, 999]
-    std::uint16_t microsecond; // [0, 999]
-    std::uint16_t nanosecond;  // [0, 999]
-
-    local_time(int h, int m, int s,
-               int ms = 0, int us = 0, int ns = 0)
-        : hour  (static_cast<std::uint8_t>(h)),
-          minute(static_cast<std::uint8_t>(m)),
-          second(static_cast<std::uint8_t>(s)),
-          millisecond(static_cast<std::uint16_t>(ms)),
-          microsecond(static_cast<std::uint16_t>(us)),
-          nanosecond (static_cast<std::uint16_t>(ns))
-    {}
-
-    explicit local_time(const std::tm& t)
-        : hour  (static_cast<std::uint8_t>(t.tm_hour)),
-          minute(static_cast<std::uint8_t>(t.tm_min)),
-          second(static_cast<std::uint8_t>(t.tm_sec)),
-          millisecond(0), microsecond(0), nanosecond(0)
-    {}
-
-    template<typename Rep, typename Period>
-    explicit local_time(const std::chrono::duration<Rep, Period>& t)
-    {
-        const auto h = std::chrono::duration_cast<std::chrono::hours>(t);
-        this->hour = static_cast<std::uint8_t>(h.count());
-        const auto t2 = t - h;
-        const auto m = std::chrono::duration_cast<std::chrono::minutes>(t2);
-        this->minute = static_cast<std::uint8_t>(m.count());
-        const auto t3 = t2 - m;
-        const auto s = std::chrono::duration_cast<std::chrono::seconds>(t3);
-        this->second = static_cast<std::uint8_t>(s.count());
-        const auto t4 = t3 - s;
-        const auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(t4);
-        this->millisecond = static_cast<std::uint16_t>(ms.count());
-        const auto t5 = t4 - ms;
-        const auto us = std::chrono::duration_cast<std::chrono::microseconds>(t5);
-        this->microsecond = static_cast<std::uint16_t>(us.count());
-        const auto t6 = t5 - us;
-        const auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(t6);
-        this->nanosecond = static_cast<std::uint16_t>(ns.count());
-    }
-
-    operator std::chrono::nanoseconds() const
-    {
-        return std::chrono::nanoseconds (this->nanosecond)  +
-               std::chrono::microseconds(this->microsecond) +
-               std::chrono::milliseconds(this->millisecond) +
-               std::chrono::seconds(this->second) +
-               std::chrono::minutes(this->minute) +
-               std::chrono::hours(this->hour);
-    }
-
-    local_time() = default;
-    ~local_time() = default;
-    local_time(local_time const&) = default;
-    local_time(local_time&&)      = default;
-    local_time& operator=(local_time const&) = default;
-    local_time& operator=(local_time&&)      = default;
-};
-
-inline bool operator==(const local_time& lhs, const local_time& rhs)
-{
-    return std::make_tuple(lhs.hour, lhs.minute, lhs.second, lhs.millisecond, lhs.microsecond, lhs.nanosecond) ==
-           std::make_tuple(rhs.hour, rhs.minute, rhs.second, rhs.millisecond, rhs.microsecond, rhs.nanosecond);
-}
-inline bool operator!=(const local_time& lhs, const local_time& rhs)
-{
-    return !(lhs == rhs);
-}
-inline bool operator< (const local_time& lhs, const local_time& rhs)
-{
-    return std::make_tuple(lhs.hour, lhs.minute, lhs.second, lhs.millisecond, lhs.microsecond, lhs.nanosecond) <
-           std::make_tuple(rhs.hour, rhs.minute, rhs.second, rhs.millisecond, rhs.microsecond, rhs.nanosecond);
-}
-inline bool operator<=(const local_time& lhs, const local_time& rhs)
-{
-    return (lhs < rhs) || (lhs == rhs);
-}
-inline bool operator> (const local_time& lhs, const local_time& rhs)
-{
-    return !(lhs <= rhs);
-}
-inline bool operator>=(const local_time& lhs, const local_time& rhs)
-{
-    return !(lhs < rhs);
-}
-
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, const local_time& time)
-{
-    os << std::setfill('0') << std::setw(2) << static_cast<int>(time.hour  ) << ':';
-    os << std::setfill('0') << std::setw(2) << static_cast<int>(time.minute) << ':';
-    os << std::setfill('0') << std::setw(2) << static_cast<int>(time.second);
-    if(time.millisecond != 0 || time.microsecond != 0 || time.nanosecond != 0)
-    {
-        os << '.';
-        os << std::setfill('0') << std::setw(3) << static_cast<int>(time.millisecond);
-        if(time.microsecond != 0 || time.nanosecond != 0)
-        {
-            os << std::setfill('0') << std::setw(3) << static_cast<int>(time.microsecond);
-            if(time.nanosecond != 0)
-            {
-                os << std::setfill('0') << std::setw(3) << static_cast<int>(time.nanosecond);
-            }
-        }
-    }
-    return os;
-}
-
-struct time_offset
-{
-    std::int8_t hour;   // [-12, 12]
-    std::int8_t minute; // [-59, 59]
-
-    time_offset(int h, int m)
-        : hour  (static_cast<std::int8_t>(h)),
-          minute(static_cast<std::int8_t>(m))
-    {}
-
-    operator std::chrono::minutes() const
-    {
-        return std::chrono::minutes(this->minute) +
-               std::chrono::hours(this->hour);
-    }
-
-    time_offset() = default;
-    ~time_offset() = default;
-    time_offset(time_offset const&) = default;
-    time_offset(time_offset&&)      = default;
-    time_offset& operator=(time_offset const&) = default;
-    time_offset& operator=(time_offset&&)      = default;
-};
-
-inline bool operator==(const time_offset& lhs, const time_offset& rhs)
-{
-    return std::make_tuple(lhs.hour, lhs.minute) ==
-           std::make_tuple(rhs.hour, rhs.minute);
-}
-inline bool operator!=(const time_offset& lhs, const time_offset& rhs)
-{
-    return !(lhs == rhs);
-}
-inline bool operator< (const time_offset& lhs, const time_offset& rhs)
-{
-    return std::make_tuple(lhs.hour, lhs.minute) <
-           std::make_tuple(rhs.hour, rhs.minute);
-}
-inline bool operator<=(const time_offset& lhs, const time_offset& rhs)
-{
-    return (lhs < rhs) || (lhs == rhs);
-}
-inline bool operator> (const time_offset& lhs, const time_offset& rhs)
-{
-    return !(lhs <= rhs);
-}
-inline bool operator>=(const time_offset& lhs, const time_offset& rhs)
-{
-    return !(lhs < rhs);
-}
-
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, const time_offset& offset)
-{
-    if(offset.hour == 0 && offset.minute == 0)
-    {
-        os << 'Z';
-        return os;
-    }
-    int minute = static_cast<int>(offset.hour) * 60 + offset.minute;
-    if(minute < 0){os << '-'; minute = std::abs(minute);} else {os << '+';}
-    os << std::setfill('0') << std::setw(2) << minute / 60 << ':';
-    os << std::setfill('0') << std::setw(2) << minute % 60;
-    return os;
-}
-
-struct local_datetime
-{
-    local_date date;
-    local_time time;
-
-    local_datetime(local_date d, local_time t): date(d), time(t) {}
-
-    explicit local_datetime(const std::tm& t): date(t), time(t){}
-
-    explicit local_datetime(const std::chrono::system_clock::time_point& tp)
-    {
-        const auto t = std::chrono::system_clock::to_time_t(tp);
-        std::tm ltime = detail::localtime_s(&t);
-
-        this->date = local_date(ltime);
-        this->time = local_time(ltime);
-
-        // std::tm lacks subsecond information, so diff between tp and tm
-        // can be used to get millisecond & microsecond information.
-        const auto t_diff = tp -
-            std::chrono::system_clock::from_time_t(std::mktime(&ltime));
-        this->time.millisecond = static_cast<std::uint16_t>(
-          std::chrono::duration_cast<std::chrono::milliseconds>(t_diff).count());
-        this->time.microsecond = static_cast<std::uint16_t>(
-          std::chrono::duration_cast<std::chrono::microseconds>(t_diff).count());
-        this->time.nanosecond = static_cast<std::uint16_t>(
-          std::chrono::duration_cast<std::chrono::nanoseconds >(t_diff).count());
-    }
-
-    explicit local_datetime(const std::time_t t)
-        : local_datetime(std::chrono::system_clock::from_time_t(t))
-    {}
-
-    operator std::chrono::system_clock::time_point() const
-    {
-        using internal_duration =
-            typename std::chrono::system_clock::time_point::duration;
-
-        // Normally DST begins at A.M. 3 or 4. If we re-use conversion operator
-        // of local_date and local_time independently, the conversion fails if
-        // it is the day when DST begins or ends. Since local_date considers the
-        // time is 00:00 A.M. and local_time does not consider DST because it
-        // does not have any date information. We need to consider both date and
-        // time information at the same time to convert it correctly.
-
-        std::tm t;
-        t.tm_sec   = static_cast<int>(this->time.second);
-        t.tm_min   = static_cast<int>(this->time.minute);
-        t.tm_hour  = static_cast<int>(this->time.hour);
-        t.tm_mday  = static_cast<int>(this->date.day);
-        t.tm_mon   = static_cast<int>(this->date.month);
-        t.tm_year  = static_cast<int>(this->date.year) - 1900;
-        t.tm_wday  = 0; // the value will be ignored
-        t.tm_yday  = 0; // the value will be ignored
-        t.tm_isdst = -1;
-
-        // std::mktime returns date as local time zone. no conversion needed
-        auto dt = std::chrono::system_clock::from_time_t(std::mktime(&t));
-        dt += std::chrono::duration_cast<internal_duration>(
-                std::chrono::milliseconds(this->time.millisecond) +
-                std::chrono::microseconds(this->time.microsecond) +
-                std::chrono::nanoseconds (this->time.nanosecond));
-        return dt;
-    }
-
-    operator std::time_t() const
-    {
-        return std::chrono::system_clock::to_time_t(
-                std::chrono::system_clock::time_point(*this));
-    }
-
-    local_datetime() = default;
-    ~local_datetime() = default;
-    local_datetime(local_datetime const&) = default;
-    local_datetime(local_datetime&&)      = default;
-    local_datetime& operator=(local_datetime const&) = default;
-    local_datetime& operator=(local_datetime&&)      = default;
-};
-
-inline bool operator==(const local_datetime& lhs, const local_datetime& rhs)
-{
-    return std::make_tuple(lhs.date, lhs.time) ==
-           std::make_tuple(rhs.date, rhs.time);
-}
-inline bool operator!=(const local_datetime& lhs, const local_datetime& rhs)
-{
-    return !(lhs == rhs);
-}
-inline bool operator< (const local_datetime& lhs, const local_datetime& rhs)
-{
-    return std::make_tuple(lhs.date, lhs.time) <
-           std::make_tuple(rhs.date, rhs.time);
-}
-inline bool operator<=(const local_datetime& lhs, const local_datetime& rhs)
-{
-    return (lhs < rhs) || (lhs == rhs);
-}
-inline bool operator> (const local_datetime& lhs, const local_datetime& rhs)
-{
-    return !(lhs <= rhs);
-}
-inline bool operator>=(const local_datetime& lhs, const local_datetime& rhs)
-{
-    return !(lhs < rhs);
-}
-
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, const local_datetime& dt)
-{
-    os << dt.date << 'T' << dt.time;
-    return os;
-}
-
-struct offset_datetime
-{
-    local_date  date;
-    local_time  time;
-    time_offset offset;
-
-    offset_datetime(local_date d, local_time t, time_offset o)
-        : date(d), time(t), offset(o)
-    {}
-    offset_datetime(const local_datetime& dt, time_offset o)
-        : date(dt.date), time(dt.time), offset(o)
-    {}
-    explicit offset_datetime(const local_datetime& ld)
-        : date(ld.date), time(ld.time), offset(get_local_offset(nullptr))
-          // use the current local timezone offset
-    {}
-    explicit offset_datetime(const std::chrono::system_clock::time_point& tp)
-        : offset(0, 0) // use gmtime
-    {
-        const auto timet = std::chrono::system_clock::to_time_t(tp);
-        const auto tm    = detail::gmtime_s(&timet);
-        this->date = local_date(tm);
-        this->time = local_time(tm);
-    }
-    explicit offset_datetime(const std::time_t& t)
-        : offset(0, 0) // use gmtime
-    {
-        const auto tm    = detail::gmtime_s(&t);
-        this->date = local_date(tm);
-        this->time = local_time(tm);
-    }
-    explicit offset_datetime(const std::tm& t)
-        : offset(0, 0) // assume gmtime
-    {
-        this->date = local_date(t);
-        this->time = local_time(t);
-    }
-
-    operator std::chrono::system_clock::time_point() const
-    {
-        // get date-time
-        using internal_duration =
-            typename std::chrono::system_clock::time_point::duration;
-
-        // first, convert it to local date-time information in the same way as
-        // local_datetime does. later we will use time_t to adjust time offset.
-        std::tm t;
-        t.tm_sec   = static_cast<int>(this->time.second);
-        t.tm_min   = static_cast<int>(this->time.minute);
-        t.tm_hour  = static_cast<int>(this->time.hour);
-        t.tm_mday  = static_cast<int>(this->date.day);
-        t.tm_mon   = static_cast<int>(this->date.month);
-        t.tm_year  = static_cast<int>(this->date.year) - 1900;
-        t.tm_wday  = 0; // the value will be ignored
-        t.tm_yday  = 0; // the value will be ignored
-        t.tm_isdst = -1;
-        const std::time_t tp_loc = std::mktime(std::addressof(t));
-
-        auto tp = std::chrono::system_clock::from_time_t(tp_loc);
-        tp += std::chrono::duration_cast<internal_duration>(
-                std::chrono::milliseconds(this->time.millisecond) +
-                std::chrono::microseconds(this->time.microsecond) +
-                std::chrono::nanoseconds (this->time.nanosecond));
-
-        // Since mktime uses local time zone, it should be corrected.
-        // `12:00:00+09:00` means `03:00:00Z`. So mktime returns `03:00:00Z` if
-        // we are in `+09:00` timezone. To represent `12:00:00Z` there, we need
-        // to add `+09:00` to `03:00:00Z`.
-        //    Here, it uses the time_t converted from date-time info to handle
-        // daylight saving time.
-        const auto ofs = get_local_offset(std::addressof(tp_loc));
-        tp += std::chrono::hours  (ofs.hour);
-        tp += std::chrono::minutes(ofs.minute);
-
-        // We got `12:00:00Z` by correcting local timezone applied by mktime.
-        // Then we will apply the offset. Let's say `12:00:00-08:00` is given.
-        // And now, we have `12:00:00Z`. `12:00:00-08:00` means `20:00:00Z`.
-        // So we need to subtract the offset.
-        tp -= std::chrono::minutes(this->offset);
-        return tp;
-    }
-
-    operator std::time_t() const
-    {
-        return std::chrono::system_clock::to_time_t(
-                std::chrono::system_clock::time_point(*this));
-    }
-
-    offset_datetime() = default;
-    ~offset_datetime() = default;
-    offset_datetime(offset_datetime const&) = default;
-    offset_datetime(offset_datetime&&)      = default;
-    offset_datetime& operator=(offset_datetime const&) = default;
-    offset_datetime& operator=(offset_datetime&&)      = default;
-
-  private:
-
-    static time_offset get_local_offset(const std::time_t* tp)
-    {
-        // get local timezone with the same date-time information as mktime
-        const auto t = detail::localtime_s(tp);
-
-        std::array<char, 6> buf;
-        const auto result = std::strftime(buf.data(), 6, "%z", &t); // +hhmm\0
-        if(result != 5)
-        {
-            throw std::runtime_error("toml::offset_datetime: cannot obtain "
-                                     "timezone information of current env");
-        }
-        const int ofs = std::atoi(buf.data());
-        const int ofs_h = ofs / 100;
-        const int ofs_m = ofs - (ofs_h * 100);
-        return time_offset(ofs_h, ofs_m);
-    }
-};
-
-inline bool operator==(const offset_datetime& lhs, const offset_datetime& rhs)
-{
-    return std::make_tuple(lhs.date, lhs.time, lhs.offset) ==
-           std::make_tuple(rhs.date, rhs.time, rhs.offset);
-}
-inline bool operator!=(const offset_datetime& lhs, const offset_datetime& rhs)
-{
-    return !(lhs == rhs);
-}
-inline bool operator< (const offset_datetime& lhs, const offset_datetime& rhs)
-{
-    return std::make_tuple(lhs.date, lhs.time, lhs.offset) <
-           std::make_tuple(rhs.date, rhs.time, rhs.offset);
-}
-inline bool operator<=(const offset_datetime& lhs, const offset_datetime& rhs)
-{
-    return (lhs < rhs) || (lhs == rhs);
-}
-inline bool operator> (const offset_datetime& lhs, const offset_datetime& rhs)
-{
-    return !(lhs <= rhs);
-}
-inline bool operator>=(const offset_datetime& lhs, const offset_datetime& rhs)
-{
-    return !(lhs < rhs);
-}
-
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, const offset_datetime& dt)
-{
-    os << dt.date << 'T' << dt.time << dt.offset;
-    return os;
-}
-
-}//toml
-#endif// TOML11_DATETIME
diff --git a/src/frontend/qt_sdl/toml/toml/exception.hpp b/src/frontend/qt_sdl/toml/toml/exception.hpp
deleted file mode 100644
index c64651d0..00000000
--- a/src/frontend/qt_sdl/toml/toml/exception.hpp
+++ /dev/null
@@ -1,65 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_EXCEPTION_HPP
-#define TOML11_EXCEPTION_HPP
-#include <stdexcept>
-#include <string>
-
-#include "source_location.hpp"
-
-namespace toml
-{
-
-struct exception : public std::exception
-{
-  public:
-    explicit exception(const source_location& loc): loc_(loc) {}
-    virtual ~exception() noexcept override = default;
-    virtual const char* what() const noexcept override {return "";}
-    virtual source_location const& location() const noexcept {return loc_;}
-
-  protected:
-    source_location loc_;
-};
-
-struct syntax_error : public toml::exception
-{
-  public:
-    explicit syntax_error(const std::string& what_arg, const source_location& loc)
-        : exception(loc), what_(what_arg)
-    {}
-    virtual ~syntax_error() noexcept override = default;
-    virtual const char* what() const noexcept override {return what_.c_str();}
-
-  protected:
-    std::string what_;
-};
-
-struct type_error : public toml::exception
-{
-  public:
-    explicit type_error(const std::string& what_arg, const source_location& loc)
-        : exception(loc), what_(what_arg)
-    {}
-    virtual ~type_error() noexcept override = default;
-    virtual const char* what() const noexcept override {return what_.c_str();}
-
-  protected:
-    std::string what_;
-};
-
-struct internal_error : public toml::exception
-{
-  public:
-    explicit internal_error(const std::string& what_arg, const source_location& loc)
-        : exception(loc), what_(what_arg)
-    {}
-    virtual ~internal_error() noexcept override = default;
-    virtual const char* what() const noexcept override {return what_.c_str();}
-
-  protected:
-    std::string what_;
-};
-
-} // toml
-#endif // TOML_EXCEPTION
diff --git a/src/frontend/qt_sdl/toml/toml/get.hpp b/src/frontend/qt_sdl/toml/toml/get.hpp
deleted file mode 100644
index 901790b5..00000000
--- a/src/frontend/qt_sdl/toml/toml/get.hpp
+++ /dev/null
@@ -1,1119 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_GET_HPP
-#define TOML11_GET_HPP
-#include <algorithm>
-
-#include "from.hpp"
-#include "result.hpp"
-#include "value.hpp"
-
-namespace toml
-{
-
-// ============================================================================
-// exact toml::* type
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T> &
-get(basic_value<C, M, V>& v)
-{
-    return v.template cast<detail::type_to_enum<T, basic_value<C, M, V>>::value>();
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T> const&
-get(const basic_value<C, M, V>& v)
-{
-    return v.template cast<detail::type_to_enum<T, basic_value<C, M, V>>::value>();
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T>
-get(basic_value<C, M, V>&& v)
-{
-    return T(std::move(v).template cast<detail::type_to_enum<T, basic_value<C, M, V>>::value>());
-}
-
-// ============================================================================
-// T == toml::value; identity transformation.
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<std::is_same<T, basic_value<C, M, V>>::value, T>&
-get(basic_value<C, M, V>& v)
-{
-    return v;
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<std::is_same<T, basic_value<C, M, V>>::value, T> const&
-get(const basic_value<C, M, V>& v)
-{
-    return v;
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<std::is_same<T, basic_value<C, M, V>>::value, T>
-get(basic_value<C, M, V>&& v)
-{
-    return basic_value<C, M, V>(std::move(v));
-}
-
-// ============================================================================
-// T == toml::basic_value<C2, M2, V2>; basic_value -> basic_value
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<detail::conjunction<detail::is_basic_value<T>,
-    detail::negation<std::is_same<T, basic_value<C, M, V>>>
-    >::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    return T(v);
-}
-
-// ============================================================================
-// integer convertible from toml::Integer
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<detail::conjunction<
-    std::is_integral<T>,                            // T is integral
-    detail::negation<std::is_same<T, bool>>,        // but not bool
-    detail::negation<                               // but not toml::integer
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
-    >::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    return static_cast<T>(v.as_integer());
-}
-
-// ============================================================================
-// floating point convertible from toml::Float
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<detail::conjunction<
-    std::is_floating_point<T>,                      // T is floating_point
-    detail::negation<                               // but not toml::floating
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
-    >::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    return static_cast<T>(v.as_floating());
-}
-
-// ============================================================================
-// std::string; toml uses its own toml::string, but it should be convertible to
-// std::string seamlessly
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<std::is_same<T, std::string>::value, std::string>&
-get(basic_value<C, M, V>& v)
-{
-    return v.as_string().str;
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<std::is_same<T, std::string>::value, std::string> const&
-get(const basic_value<C, M, V>& v)
-{
-    return v.as_string().str;
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<std::is_same<T, std::string>::value, std::string>
-get(basic_value<C, M, V>&& v)
-{
-    return std::string(std::move(v.as_string().str));
-}
-
-// ============================================================================
-// std::string_view
-
-#if defined(TOML11_USING_STRING_VIEW) && TOML11_USING_STRING_VIEW>0
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<std::is_same<T, std::string_view>::value, std::string_view>
-get(const basic_value<C, M, V>& v)
-{
-    return std::string_view(v.as_string().str);
-}
-#endif
-
-// ============================================================================
-// std::chrono::duration from toml::local_time.
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<detail::is_chrono_duration<T>::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    return std::chrono::duration_cast<T>(
-            std::chrono::nanoseconds(v.as_local_time()));
-}
-
-// ============================================================================
-// std::chrono::system_clock::time_point from toml::datetime variants
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-inline detail::enable_if_t<
-    std::is_same<std::chrono::system_clock::time_point, T>::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    switch(v.type())
-    {
-        case value_t::local_date:
-        {
-            return std::chrono::system_clock::time_point(v.as_local_date());
-        }
-        case value_t::local_datetime:
-        {
-            return std::chrono::system_clock::time_point(v.as_local_datetime());
-        }
-        case value_t::offset_datetime:
-        {
-            return std::chrono::system_clock::time_point(v.as_offset_datetime());
-        }
-        default:
-        {
-            throw type_error(detail::format_underline("toml::value: "
-                "bad_cast to std::chrono::system_clock::time_point", {
-                    {v.location(), concat_to_string("the actual type is ", v.type())}
-                }), v.location());
-        }
-    }
-}
-
-// ============================================================================
-// forward declaration to use this recursively. ignore this and go ahead.
-
-// array-like type with push_back(value) method
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::is_container<T>,         // T is a container
-    detail::has_push_back_method<T>, // T::push_back(value) works
-    detail::negation<                // but not toml::array
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
-    >::value, T>
-get(const basic_value<C, M, V>&);
-
-// array-like type without push_back(value) method
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::is_container<T>,                           // T is a container
-    detail::negation<detail::has_push_back_method<T>>, // w/o push_back(...)
-    detail::negation<detail::has_specialized_from<T>>, // T does not have special conversion
-    detail::negation<                                  // not toml::array
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
-    >::value, T>
-get(const basic_value<C, M, V>&);
-
-// std::pair<T1, T2>
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_std_pair<T>::value, T>
-get(const basic_value<C, M, V>&);
-
-// std::tuple<T1, T2, ...>
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_std_tuple<T>::value, T>
-get(const basic_value<C, M, V>&);
-
-// map-like classes
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::is_map<T>, // T is map
-    detail::negation<  // but not toml::table
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
-    >::value, T>
-get(const basic_value<C, M, V>&);
-
-// T.from_toml(v)
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::negation<                         // not a toml::* type
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>,
-    detail::has_from_toml_method<T, C, M, V>, // but has from_toml(toml::value)
-    std::is_default_constructible<T>          // and default constructible
-    >::value, T>
-get(const basic_value<C, M, V>&);
-
-// toml::from<T>::from_toml(v)
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::has_specialized_from<T>::value, T>
-get(const basic_value<C, M, V>&);
-
-// T(const toml::value&) and T is not toml::basic_value,
-// and it does not have `from<T>` nor `from_toml`.
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::negation<detail::is_basic_value<T>>,
-    std::is_constructible<T, const basic_value<C, M, V>&>,
-    detail::negation<detail::has_from_toml_method<T, C, M, V>>,
-    detail::negation<detail::has_specialized_from<T>>
-    >::value, T>
-get(const basic_value<C, M, V>&);
-
-// ============================================================================
-// array-like types; most likely STL container, like std::vector, etc.
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::is_container<T>,         // T is a container
-    detail::has_push_back_method<T>, // container.push_back(elem) works
-    detail::negation<                // but not toml::array
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
-    >::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    using value_type = typename T::value_type;
-    const auto& ary = v.as_array();
-
-    T container;
-    try_reserve(container, ary.size());
-
-    for(const auto& elem : ary)
-    {
-        container.push_back(get<value_type>(elem));
-    }
-    return container;
-}
-
-// ============================================================================
-// std::forward_list does not have push_back, insert, or emplace.
-// It has insert_after, emplace_after, push_front.
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_std_forward_list<T>::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    using value_type = typename T::value_type;
-    T container;
-    for(const auto& elem : v.as_array())
-    {
-        container.push_front(get<value_type>(elem));
-    }
-    container.reverse();
-    return container;
-}
-
-// ============================================================================
-// array-like types, without push_back(). most likely [std|boost]::array.
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::is_container<T>,                           // T is a container
-    detail::negation<detail::has_push_back_method<T>>, // w/o push_back
-    detail::negation<detail::has_specialized_from<T>>, // T does not have special conversion
-    detail::negation<                                  // T is not toml::array
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
-    >::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    using value_type = typename T::value_type;
-    const auto& ar = v.as_array();
-
-    T container;
-    if(ar.size() != container.size())
-    {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "toml::get: specified container size is ", container.size(),
-            " but there are ", ar.size(), " elements in toml array."), {
-                {v.location(), "here"}
-            }));
-    }
-    for(std::size_t i=0; i<ar.size(); ++i)
-    {
-        container[i] = ::toml::get<value_type>(ar[i]);
-    }
-    return container;
-}
-
-// ============================================================================
-// std::pair.
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_std_pair<T>::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    using first_type  = typename T::first_type;
-    using second_type = typename T::second_type;
-
-    const auto& ar = v.as_array();
-    if(ar.size() != 2)
-    {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "toml::get: specified std::pair but there are ", ar.size(),
-            " elements in toml array."), {{v.location(), "here"}}));
-    }
-    return std::make_pair(::toml::get<first_type >(ar.at(0)),
-                          ::toml::get<second_type>(ar.at(1)));
-}
-
-// ============================================================================
-// std::tuple.
-
-namespace detail
-{
-template<typename T, typename Array, std::size_t ... I>
-T get_tuple_impl(const Array& a, index_sequence<I...>)
-{
-    return std::make_tuple(
-        ::toml::get<typename std::tuple_element<I, T>::type>(a.at(I))...);
-}
-} // detail
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_std_tuple<T>::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    const auto& ar = v.as_array();
-    if(ar.size() != std::tuple_size<T>::value)
-    {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "toml::get: specified std::tuple with ",
-            std::tuple_size<T>::value, " elements, but there are ", ar.size(),
-            " elements in toml array."), {{v.location(), "here"}}));
-    }
-    return detail::get_tuple_impl<T>(ar,
-            detail::make_index_sequence<std::tuple_size<T>::value>{});
-}
-
-// ============================================================================
-// map-like types; most likely STL map, like std::map or std::unordered_map.
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::is_map<T>, // T is map
-    detail::negation<  // but not toml::array
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
-    >::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    using key_type    = typename T::key_type;
-    using mapped_type = typename T::mapped_type;
-    static_assert(std::is_convertible<std::string, key_type>::value,
-                  "toml::get only supports map type of which key_type is "
-                  "convertible from std::string.");
-    T map;
-    for(const auto& kv : v.as_table())
-    {
-        map.emplace(key_type(kv.first), get<mapped_type>(kv.second));
-    }
-    return map;
-}
-
-// ============================================================================
-// user-defined, but compatible types.
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::negation<                         // not a toml::* type
-        detail::is_exact_toml_type<T, basic_value<C, M, V>>>,
-    detail::has_from_toml_method<T, C, M, V>, // but has from_toml(toml::value) memfn
-    std::is_default_constructible<T>          // and default constructible
-    >::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    T ud;
-    ud.from_toml(v);
-    return ud;
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::has_specialized_from<T>::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    return ::toml::from<T>::from_toml(v);
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::negation<detail::is_basic_value<T>>,                // T is not a toml::value
-    std::is_constructible<T, const basic_value<C, M, V>&>,      // T is constructible from toml::value
-    detail::negation<detail::has_from_toml_method<T, C, M, V>>, // and T does not have T.from_toml(v);
-    detail::negation<detail::has_specialized_from<T>>           // and T does not have toml::from<T>{};
-    >::value, T>
-get(const basic_value<C, M, V>& v)
-{
-    return T(v);
-}
-
-// ============================================================================
-// find
-
-// ----------------------------------------------------------------------------
-// these overloads do not require to set T. and returns value itself.
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V> const& find(const basic_value<C, M, V>& v, const key& ky)
-{
-    const auto& tab = v.as_table();
-    if(tab.count(ky) == 0)
-    {
-        detail::throw_key_not_found_error(v, ky);
-    }
-    return tab.at(ky);
-}
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V>& find(basic_value<C, M, V>& v, const key& ky)
-{
-    auto& tab = v.as_table();
-    if(tab.count(ky) == 0)
-    {
-        detail::throw_key_not_found_error(v, ky);
-    }
-    return tab.at(ky);
-}
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V> find(basic_value<C, M, V>&& v, const key& ky)
-{
-    typename basic_value<C, M, V>::table_type tab = std::move(v).as_table();
-    if(tab.count(ky) == 0)
-    {
-        detail::throw_key_not_found_error(v, ky);
-    }
-    return basic_value<C, M, V>(std::move(tab.at(ky)));
-}
-
-// ----------------------------------------------------------------------------
-// find(value, idx)
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V> const&
-find(const basic_value<C, M, V>& v, const std::size_t idx)
-{
-    const auto& ary = v.as_array();
-    if(ary.size() <= idx)
-    {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
-    }
-    return ary.at(idx);
-}
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V>& find(basic_value<C, M, V>& v, const std::size_t idx)
-{
-    auto& ary = v.as_array();
-    if(ary.size() <= idx)
-    {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
-    }
-    return ary.at(idx);
-}
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V> find(basic_value<C, M, V>&& v, const std::size_t idx)
-{
-    auto& ary = v.as_array();
-    if(ary.size() <= idx)
-    {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
-    }
-    return basic_value<C, M, V>(std::move(ary.at(idx)));
-}
-
-// ----------------------------------------------------------------------------
-// find<T>(value, key);
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-decltype(::toml::get<T>(std::declval<basic_value<C, M, V> const&>()))
-find(const basic_value<C, M, V>& v, const key& ky)
-{
-    const auto& tab = v.as_table();
-    if(tab.count(ky) == 0)
-    {
-        detail::throw_key_not_found_error(v, ky);
-    }
-    return ::toml::get<T>(tab.at(ky));
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&>()))
-find(basic_value<C, M, V>& v, const key& ky)
-{
-    auto& tab = v.as_table();
-    if(tab.count(ky) == 0)
-    {
-        detail::throw_key_not_found_error(v, ky);
-    }
-    return ::toml::get<T>(tab.at(ky));
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&&>()))
-find(basic_value<C, M, V>&& v, const key& ky)
-{
-    typename basic_value<C, M, V>::table_type tab = std::move(v).as_table();
-    if(tab.count(ky) == 0)
-    {
-        detail::throw_key_not_found_error(v, ky);
-    }
-    return ::toml::get<T>(std::move(tab.at(ky)));
-}
-
-// ----------------------------------------------------------------------------
-// find<T>(value, idx)
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-decltype(::toml::get<T>(std::declval<basic_value<C, M, V> const&>()))
-find(const basic_value<C, M, V>& v, const std::size_t idx)
-{
-    const auto& ary = v.as_array();
-    if(ary.size() <= idx)
-    {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
-    }
-    return ::toml::get<T>(ary.at(idx));
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&>()))
-find(basic_value<C, M, V>& v, const std::size_t idx)
-{
-    auto& ary = v.as_array();
-    if(ary.size() <= idx)
-    {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
-    }
-    return ::toml::get<T>(ary.at(idx));
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&&>()))
-find(basic_value<C, M, V>&& v, const std::size_t idx)
-{
-    typename basic_value<C, M, V>::array_type ary = std::move(v).as_array();
-    if(ary.size() <= idx)
-    {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
-    }
-    return ::toml::get<T>(std::move(ary.at(idx)));
-}
-
-// --------------------------------------------------------------------------
-// toml::find(toml::value, toml::key, Ts&& ... keys)
-
-namespace detail
-{
-// It suppresses warnings by -Wsign-conversion. Let's say we have the following
-// code.
-// ```cpp
-// const auto x = toml::find<std::string>(data, "array", 0);
-// ```
-// Here, the type of literal number `0` is `int`. `int` is a signed integer.
-// `toml::find` takes `std::size_t` as an index. So it causes implicit sign
-// conversion and `-Wsign-conversion` warns about it. Using `0u` instead of `0`
-// suppresses the warning, but it makes user code messy.
-//     To suppress this warning, we need to be aware of type conversion caused
-// by `toml::find(v, key1, key2, ... keys)`. But the thing is that the types of
-// keys can be any combination of {string-like, size_t-like}. Of course we can't
-// write down all the combinations. Thus we need to use some function that
-// recognize the type of argument and cast it into `std::string` or
-// `std::size_t` depending on the context.
-//     `key_cast` does the job. It has 2 overloads. One is invoked when the
-// argument type is an integer and cast the argument into `std::size_t`. The
-// other is invoked when the argument type is not an integer, possibly one of
-// std::string, const char[N] or const char*, and construct std::string from
-// the argument.
-//     `toml::find(v, k1, k2, ... ks)` uses `key_cast` before passing `ks` to
-// `toml::find(v, k)` to suppress -Wsign-conversion.
-
-template<typename T>
-enable_if_t<conjunction<std::is_integral<remove_cvref_t<T>>,
-            negation<std::is_same<remove_cvref_t<T>, bool>>>::value, std::size_t>
-key_cast(T&& v) noexcept
-{
-    return std::size_t(v);
-}
-template<typename T>
-enable_if_t<negation<conjunction<std::is_integral<remove_cvref_t<T>>,
-            negation<std::is_same<remove_cvref_t<T>, bool>>>>::value, std::string>
-key_cast(T&& v) noexcept
-{
-    return std::string(std::forward<T>(v));
-}
-} // detail
-
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V,
-         typename Key1, typename Key2, typename ... Keys>
-const basic_value<C, M, V>&
-find(const basic_value<C, M, V>& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
-{
-    return ::toml::find(::toml::find(v, detail::key_cast(k1)),
-            detail::key_cast(k2), std::forward<Keys>(keys)...);
-}
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V,
-         typename Key1, typename Key2, typename ... Keys>
-basic_value<C, M, V>&
-find(basic_value<C, M, V>& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
-{
-    return ::toml::find(::toml::find(v, detail::key_cast(k1)),
-            detail::key_cast(k2), std::forward<Keys>(keys)...);
-}
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V,
-         typename Key1, typename Key2, typename ... Keys>
-basic_value<C, M, V>
-find(basic_value<C, M, V>&& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
-{
-    return ::toml::find(::toml::find(std::move(v), std::forward<Key1>(k1)),
-            detail::key_cast(k2), std::forward<Keys>(keys)...);
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V,
-         typename Key1, typename Key2, typename ... Keys>
-decltype(::toml::get<T>(std::declval<const basic_value<C, M, V>&>()))
-find(const basic_value<C, M, V>& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
-{
-    return ::toml::find<T>(::toml::find(v, detail::key_cast(k1)),
-            detail::key_cast(k2), std::forward<Keys>(keys)...);
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V,
-         typename Key1, typename Key2, typename ... Keys>
-decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&>()))
-find(basic_value<C, M, V>& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
-{
-    return ::toml::find<T>(::toml::find(v, detail::key_cast(k1)),
-            detail::key_cast(k2), std::forward<Keys>(keys)...);
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V,
-         typename Key1, typename Key2, typename ... Keys>
-decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&&>()))
-find(basic_value<C, M, V>&& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
-{
-    return ::toml::find<T>(::toml::find(std::move(v), detail::key_cast(k1)),
-            detail::key_cast(k2), std::forward<Keys>(keys)...);
-}
-
-// ============================================================================
-// get_or(value, fallback)
-
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V> const&
-get_or(const basic_value<C, M, V>& v, const basic_value<C, M, V>&)
-{
-    return v;
-}
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V>&
-get_or(basic_value<C, M, V>& v, basic_value<C, M, V>&)
-{
-    return v;
-}
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V>
-get_or(basic_value<C, M, V>&& v, basic_value<C, M, V>&&)
-{
-    return v;
-}
-
-// ----------------------------------------------------------------------------
-// specialization for the exact toml types (return type becomes lvalue ref)
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<
-    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T> const&
-get_or(const basic_value<C, M, V>& v, const T& opt)
-{
-    try
-    {
-        return get<detail::remove_cvref_t<T>>(v);
-    }
-    catch(...)
-    {
-        return opt;
-    }
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<
-    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T>&
-get_or(basic_value<C, M, V>& v, T& opt)
-{
-    try
-    {
-        return get<detail::remove_cvref_t<T>>(v);
-    }
-    catch(...)
-    {
-        return opt;
-    }
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_exact_toml_type<detail::remove_cvref_t<T>,
-    basic_value<C, M, V>>::value, detail::remove_cvref_t<T>>
-get_or(basic_value<C, M, V>&& v, T&& opt)
-{
-    try
-    {
-        return get<detail::remove_cvref_t<T>>(std::move(v));
-    }
-    catch(...)
-    {
-        return detail::remove_cvref_t<T>(std::forward<T>(opt));
-    }
-}
-
-// ----------------------------------------------------------------------------
-// specialization for std::string (return type becomes lvalue ref)
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<std::is_same<detail::remove_cvref_t<T>, std::string>::value,
-    std::string> const&
-get_or(const basic_value<C, M, V>& v, const T& opt)
-{
-    try
-    {
-        return v.as_string().str;
-    }
-    catch(...)
-    {
-        return opt;
-    }
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<std::is_same<T, std::string>::value, std::string>&
-get_or(basic_value<C, M, V>& v, T& opt)
-{
-    try
-    {
-        return v.as_string().str;
-    }
-    catch(...)
-    {
-        return opt;
-    }
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<
-    std::is_same<detail::remove_cvref_t<T>, std::string>::value, std::string>
-get_or(basic_value<C, M, V>&& v, T&& opt)
-{
-    try
-    {
-        return std::move(v.as_string().str);
-    }
-    catch(...)
-    {
-        return std::string(std::forward<T>(opt));
-    }
-}
-
-// ----------------------------------------------------------------------------
-// specialization for string literal
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::is_string_literal<
-    typename std::remove_reference<T>::type>::value, std::string>
-get_or(const basic_value<C, M, V>& v, T&& opt)
-{
-    try
-    {
-        return std::move(v.as_string().str);
-    }
-    catch(...)
-    {
-        return std::string(std::forward<T>(opt));
-    }
-}
-
-// ----------------------------------------------------------------------------
-// others (require type conversion and return type cannot be lvalue reference)
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    detail::negation<detail::is_exact_toml_type<detail::remove_cvref_t<T>,
-        basic_value<C, M, V>>>,
-    detail::negation<std::is_same<std::string, detail::remove_cvref_t<T>>>,
-    detail::negation<detail::is_string_literal<
-        typename std::remove_reference<T>::type>>
-    >::value, detail::remove_cvref_t<T>>
-get_or(const basic_value<C, M, V>& v, T&& opt)
-{
-    try
-    {
-        return get<detail::remove_cvref_t<T>>(v);
-    }
-    catch(...)
-    {
-        return detail::remove_cvref_t<T>(std::forward<T>(opt));
-    }
-}
-
-// ===========================================================================
-// find_or(value, key, fallback)
-
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V> const&
-find_or(const basic_value<C, M, V>& v, const key& ky,
-        const basic_value<C, M, V>& opt)
-{
-    if(!v.is_table()) {return opt;}
-    const auto& tab = v.as_table();
-    if(tab.count(ky) == 0) {return opt;}
-    return tab.at(ky);
-}
-
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V>&
-find_or(basic_value<C, M, V>& v, const toml::key& ky, basic_value<C, M, V>& opt)
-{
-    if(!v.is_table()) {return opt;}
-    auto& tab = v.as_table();
-    if(tab.count(ky) == 0) {return opt;}
-    return tab.at(ky);
-}
-
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-basic_value<C, M, V>
-find_or(basic_value<C, M, V>&& v, const toml::key& ky, basic_value<C, M, V>&& opt)
-{
-    if(!v.is_table()) {return opt;}
-    auto tab = std::move(v).as_table();
-    if(tab.count(ky) == 0) {return opt;}
-    return basic_value<C, M, V>(std::move(tab.at(ky)));
-}
-
-// ---------------------------------------------------------------------------
-// exact types (return type can be a reference)
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<
-    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T> const&
-find_or(const basic_value<C, M, V>& v, const key& ky, const T& opt)
-{
-    if(!v.is_table()) {return opt;}
-    const auto& tab = v.as_table();
-    if(tab.count(ky) == 0) {return opt;}
-    return get_or(tab.at(ky), opt);
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<
-    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T>&
-find_or(basic_value<C, M, V>& v, const toml::key& ky, T& opt)
-{
-    if(!v.is_table()) {return opt;}
-    auto& tab = v.as_table();
-    if(tab.count(ky) == 0) {return opt;}
-    return get_or(tab.at(ky), opt);
-}
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<
-    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value,
-    detail::remove_cvref_t<T>>
-find_or(basic_value<C, M, V>&& v, const toml::key& ky, T&& opt)
-{
-    if(!v.is_table()) {return std::forward<T>(opt);}
-    auto tab = std::move(v).as_table();
-    if(tab.count(ky) == 0) {return std::forward<T>(opt);}
-    return get_or(std::move(tab.at(ky)), std::forward<T>(opt));
-}
-
-// ---------------------------------------------------------------------------
-// std::string (return type can be a reference)
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<std::is_same<T, std::string>::value, std::string> const&
-find_or(const basic_value<C, M, V>& v, const key& ky, const T& opt)
-{
-    if(!v.is_table()) {return opt;}
-    const auto& tab = v.as_table();
-    if(tab.count(ky) == 0) {return opt;}
-    return get_or(tab.at(ky), opt);
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<std::is_same<T, std::string>::value, std::string>&
-find_or(basic_value<C, M, V>& v, const toml::key& ky, T& opt)
-{
-    if(!v.is_table()) {return opt;}
-    auto& tab = v.as_table();
-    if(tab.count(ky) == 0) {return opt;}
-    return get_or(tab.at(ky), opt);
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<std::is_same<T, std::string>::value, std::string>
-find_or(basic_value<C, M, V>&& v, const toml::key& ky, T&& opt)
-{
-    if(!v.is_table()) {return std::forward<T>(opt);}
-    auto tab = std::move(v).as_table();
-    if(tab.count(ky) == 0) {return std::forward<T>(opt);}
-    return get_or(std::move(tab.at(ky)), std::forward<T>(opt));
-}
-
-// ---------------------------------------------------------------------------
-// string literal (deduced as std::string)
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<
-    detail::is_string_literal<typename std::remove_reference<T>::type>::value,
-    std::string>
-find_or(const basic_value<C, M, V>& v, const toml::key& ky, T&& opt)
-{
-    if(!v.is_table()) {return std::string(opt);}
-    const auto& tab = v.as_table();
-    if(tab.count(ky) == 0) {return std::string(opt);}
-    return get_or(tab.at(ky), std::forward<T>(opt));
-}
-
-// ---------------------------------------------------------------------------
-// others (require type conversion and return type cannot be lvalue reference)
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-detail::enable_if_t<detail::conjunction<
-    // T is not an exact toml type
-    detail::negation<detail::is_exact_toml_type<
-        detail::remove_cvref_t<T>, basic_value<C, M, V>>>,
-    // T is not std::string
-    detail::negation<std::is_same<std::string, detail::remove_cvref_t<T>>>,
-    // T is not a string literal
-    detail::negation<detail::is_string_literal<
-        typename std::remove_reference<T>::type>>
-    >::value, detail::remove_cvref_t<T>>
-find_or(const basic_value<C, M, V>& v, const toml::key& ky, T&& opt)
-{
-    if(!v.is_table()) {return std::forward<T>(opt);}
-    const auto& tab = v.as_table();
-    if(tab.count(ky) == 0) {return std::forward<T>(opt);}
-    return get_or(tab.at(ky), std::forward<T>(opt));
-}
-
-// ---------------------------------------------------------------------------
-// recursive find-or with type deduction (find_or(value, keys, opt))
-
-template<typename Value, typename ... Ks,
-         typename detail::enable_if_t<(sizeof...(Ks) > 1), std::nullptr_t> = nullptr>
-         // here we need to add SFINAE in the template parameter to avoid
-         // infinite recursion in type deduction on gcc
-auto find_or(Value&& v, const toml::key& ky, Ks&& ... keys)
-    -> decltype(find_or(std::forward<Value>(v), ky, detail::last_one(std::forward<Ks>(keys)...)))
-{
-    if(!v.is_table())
-    {
-        return detail::last_one(std::forward<Ks>(keys)...);
-    }
-    auto&& tab = std::forward<Value>(v).as_table();
-    if(tab.count(ky) == 0)
-    {
-        return detail::last_one(std::forward<Ks>(keys)...);
-    }
-    return find_or(std::forward<decltype(tab)>(tab).at(ky), std::forward<Ks>(keys)...);
-}
-
-// ---------------------------------------------------------------------------
-// recursive find_or with explicit type specialization, find_or<int>(value, keys...)
-
-template<typename T, typename Value, typename ... Ks,
-         typename detail::enable_if_t<(sizeof...(Ks) > 1), std::nullptr_t> = nullptr>
-         // here we need to add SFINAE in the template parameter to avoid
-         // infinite recursion in type deduction on gcc
-auto find_or(Value&& v, const toml::key& ky, Ks&& ... keys)
-    -> decltype(find_or<T>(std::forward<Value>(v), ky, detail::last_one(std::forward<Ks>(keys)...)))
-{
-    if(!v.is_table())
-    {
-        return detail::last_one(std::forward<Ks>(keys)...);
-    }
-    auto&& tab = std::forward<Value>(v).as_table();
-    if(tab.count(ky) == 0)
-    {
-        return detail::last_one(std::forward<Ks>(keys)...);
-    }
-    return find_or(std::forward<decltype(tab)>(tab).at(ky), std::forward<Ks>(keys)...);
-}
-
-// ============================================================================
-// expect
-
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-result<T, std::string> expect(const basic_value<C, M, V>& v) noexcept
-{
-    try
-    {
-        return ok(get<T>(v));
-    }
-    catch(const std::exception& e)
-    {
-        return err(e.what());
-    }
-}
-template<typename T, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-result<T, std::string>
-expect(const basic_value<C, M, V>& v, const toml::key& k) noexcept
-{
-    try
-    {
-        return ok(find<T>(v, k));
-    }
-    catch(const std::exception& e)
-    {
-        return err(e.what());
-    }
-}
-
-} // toml
-#endif// TOML11_GET
diff --git a/src/frontend/qt_sdl/toml/toml/lexer.hpp b/src/frontend/qt_sdl/toml/toml/lexer.hpp
deleted file mode 100644
index ea5050b8..00000000
--- a/src/frontend/qt_sdl/toml/toml/lexer.hpp
+++ /dev/null
@@ -1,293 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_LEXER_HPP
-#define TOML11_LEXER_HPP
-#include <istream>
-#include <sstream>
-#include <stdexcept>
-#include <fstream>
-
-#include "combinator.hpp"
-
-namespace toml
-{
-namespace detail
-{
-
-// these scans contents from current location in a container of char
-// and extract a region that matches their own pattern.
-// to see the implementation of each component, see combinator.hpp.
-
-using lex_wschar  = either<character<' '>, character<'\t'>>;
-using lex_ws      = repeat<lex_wschar, at_least<1>>;
-using lex_newline = either<character<'\n'>,
-                           sequence<character<'\r'>, character<'\n'>>>;
-using lex_lower   = in_range<'a', 'z'>;
-using lex_upper   = in_range<'A', 'Z'>;
-using lex_alpha   = either<lex_lower, lex_upper>;
-using lex_digit   = in_range<'0', '9'>;
-using lex_nonzero = in_range<'1', '9'>;
-using lex_oct_dig = in_range<'0', '7'>;
-using lex_bin_dig = in_range<'0', '1'>;
-using lex_hex_dig = either<lex_digit, in_range<'A', 'F'>, in_range<'a', 'f'>>;
-
-using lex_hex_prefix = sequence<character<'0'>, character<'x'>>;
-using lex_oct_prefix = sequence<character<'0'>, character<'o'>>;
-using lex_bin_prefix = sequence<character<'0'>, character<'b'>>;
-using lex_underscore = character<'_'>;
-using lex_plus       = character<'+'>;
-using lex_minus      = character<'-'>;
-using lex_sign       = either<lex_plus, lex_minus>;
-
-// digit | nonzero 1*(digit | _ digit)
-using lex_unsigned_dec_int = either<sequence<lex_nonzero, repeat<
-    either<lex_digit, sequence<lex_underscore, lex_digit>>, at_least<1>>>,
-    lex_digit>;
-// (+|-)? unsigned_dec_int
-using lex_dec_int = sequence<maybe<lex_sign>, lex_unsigned_dec_int>;
-
-// hex_prefix hex_dig *(hex_dig | _ hex_dig)
-using lex_hex_int = sequence<lex_hex_prefix, sequence<lex_hex_dig, repeat<
-    either<lex_hex_dig, sequence<lex_underscore, lex_hex_dig>>, unlimited>>>;
-// oct_prefix oct_dig *(oct_dig | _ oct_dig)
-using lex_oct_int = sequence<lex_oct_prefix, sequence<lex_oct_dig, repeat<
-    either<lex_oct_dig, sequence<lex_underscore, lex_oct_dig>>, unlimited>>>;
-// bin_prefix bin_dig *(bin_dig | _ bin_dig)
-using lex_bin_int = sequence<lex_bin_prefix, sequence<lex_bin_dig, repeat<
-    either<lex_bin_dig, sequence<lex_underscore, lex_bin_dig>>, unlimited>>>;
-
-// (dec_int | hex_int | oct_int | bin_int)
-using lex_integer = either<lex_bin_int, lex_oct_int, lex_hex_int, lex_dec_int>;
-
-// ===========================================================================
-
-using lex_inf = sequence<character<'i'>, character<'n'>, character<'f'>>;
-using lex_nan = sequence<character<'n'>, character<'a'>, character<'n'>>;
-using lex_special_float = sequence<maybe<lex_sign>, either<lex_inf, lex_nan>>;
-
-using lex_zero_prefixable_int = sequence<lex_digit, repeat<either<lex_digit,
-    sequence<lex_underscore, lex_digit>>, unlimited>>;
-
-using lex_fractional_part = sequence<character<'.'>, lex_zero_prefixable_int>;
-
-using lex_exponent_part   = sequence<either<character<'e'>, character<'E'>>,
-        maybe<lex_sign>, lex_zero_prefixable_int>;
-
-using lex_float = either<lex_special_float,
-      sequence<lex_dec_int, either<lex_exponent_part,
-      sequence<lex_fractional_part, maybe<lex_exponent_part>>>>>;
-
-// ===========================================================================
-
-using lex_true = sequence<character<'t'>, character<'r'>,
-                          character<'u'>, character<'e'>>;
-using lex_false = sequence<character<'f'>, character<'a'>, character<'l'>,
-                           character<'s'>, character<'e'>>;
-using lex_boolean = either<lex_true, lex_false>;
-
-// ===========================================================================
-
-using lex_date_fullyear = repeat<lex_digit, exactly<4>>;
-using lex_date_month    = repeat<lex_digit, exactly<2>>;
-using lex_date_mday     = repeat<lex_digit, exactly<2>>;
-using lex_time_delim    = either<character<'T'>, character<'t'>, character<' '>>;
-using lex_time_hour     = repeat<lex_digit, exactly<2>>;
-using lex_time_minute   = repeat<lex_digit, exactly<2>>;
-using lex_time_second   = repeat<lex_digit, exactly<2>>;
-using lex_time_secfrac  = sequence<character<'.'>,
-                                   repeat<lex_digit, at_least<1>>>;
-
-using lex_time_numoffset = sequence<either<character<'+'>, character<'-'>>,
-                                    sequence<lex_time_hour, character<':'>,
-                                             lex_time_minute>>;
-using lex_time_offset = either<character<'Z'>, character<'z'>,
-                               lex_time_numoffset>;
-
-using lex_partial_time = sequence<lex_time_hour,   character<':'>,
-                                  lex_time_minute, character<':'>,
-                                  lex_time_second, maybe<lex_time_secfrac>>;
-using lex_full_date    = sequence<lex_date_fullyear, character<'-'>,
-                                  lex_date_month,    character<'-'>,
-                                  lex_date_mday>;
-using lex_full_time    = sequence<lex_partial_time, lex_time_offset>;
-
-using lex_offset_date_time = sequence<lex_full_date, lex_time_delim, lex_full_time>;
-using lex_local_date_time  = sequence<lex_full_date, lex_time_delim, lex_partial_time>;
-using lex_local_date       = lex_full_date;
-using lex_local_time       = lex_partial_time;
-
-// ===========================================================================
-
-using lex_quotation_mark  = character<'"'>;
-using lex_basic_unescaped = exclude<either<in_range<0x00, 0x08>, // 0x09 (tab) is allowed
-                                           in_range<0x0A, 0x1F>,
-                                           character<0x22>, character<0x5C>,
-                                           character<0x7F>>>;
-
-using lex_escape          = character<'\\'>;
-using lex_escape_unicode_short = sequence<character<'u'>,
-                                          repeat<lex_hex_dig, exactly<4>>>;
-using lex_escape_unicode_long  = sequence<character<'U'>,
-                                          repeat<lex_hex_dig, exactly<8>>>;
-using lex_escape_seq_char = either<character<'"'>, character<'\\'>,
-                                   character<'b'>, character<'f'>,
-                                   character<'n'>, character<'r'>,
-                                   character<'t'>,
-                                   lex_escape_unicode_short,
-                                   lex_escape_unicode_long
-                                   >;
-using lex_escaped      = sequence<lex_escape, lex_escape_seq_char>;
-using lex_basic_char   = either<lex_basic_unescaped, lex_escaped>;
-using lex_basic_string = sequence<lex_quotation_mark,
-                                  repeat<lex_basic_char, unlimited>,
-                                  lex_quotation_mark>;
-
-// After toml post-v0.5.0, it is explicitly clarified how quotes in ml-strings
-// are allowed to be used.
-// After this, the following strings are *explicitly* allowed.
-// - One or two `"`s in a multi-line basic string is allowed wherever it is.
-// - Three consecutive `"`s in a multi-line basic string is considered as a delimiter.
-// - One or two `"`s can appear just before or after the delimiter.
-// ```toml
-// str4 = """Here are two quotation marks: "". Simple enough."""
-// str5 = """Here are three quotation marks: ""\"."""
-// str6 = """Here are fifteen quotation marks: ""\"""\"""\"""\"""\"."""
-// str7 = """"This," she said, "is just a pointless statement.""""
-// ```
-// In the current implementation (v3.3.0), it is difficult to parse `str7` in
-// the above example. It is difficult to recognize `"` at the end of string body
-// collectly. It will be misunderstood as a `"""` delimiter and an additional,
-// invalid `"`. Like this:
-// ```console
-//   what():  [error] toml::parse_table: invalid line format
-//  --> hoge.toml
-//     |
-//  13 | str7 = """"This," she said, "is just a pointless statement.""""
-//     |                                                               ^- expected newline, but got '"'.
-// ```
-// As a quick workaround for this problem, `lex_ml_basic_string_delim` was
-// split into two, `lex_ml_basic_string_open` and `lex_ml_basic_string_close`.
-// `lex_ml_basic_string_open` allows only `"""`. `_close` allows 3-5 `"`s.
-// In parse_ml_basic_string() function, the trailing `"`s will be attached to
-// the string body.
-//
-using lex_ml_basic_string_delim = repeat<lex_quotation_mark, exactly<3>>;
-using lex_ml_basic_string_open  = lex_ml_basic_string_delim;
-using lex_ml_basic_string_close = sequence<
-        repeat<lex_quotation_mark, exactly<3>>,
-        maybe<lex_quotation_mark>, maybe<lex_quotation_mark>
-    >;
-
-using lex_ml_basic_unescaped    = exclude<either<in_range<0x00, 0x08>, // 0x09 is tab
-                                                 in_range<0x0A, 0x1F>,
-                                                 character<0x5C>, // backslash
-                                                 character<0x7F>, // DEL
-                                                 lex_ml_basic_string_delim>>;
-
-using lex_ml_basic_escaped_newline = sequence<
-        lex_escape, maybe<lex_ws>, lex_newline,
-        repeat<either<lex_ws, lex_newline>, unlimited>>;
-
-using lex_ml_basic_char = either<lex_ml_basic_unescaped, lex_escaped>;
-using lex_ml_basic_body = repeat<either<lex_ml_basic_char, lex_newline,
-                                        lex_ml_basic_escaped_newline>,
-                                 unlimited>;
-using lex_ml_basic_string = sequence<lex_ml_basic_string_open,
-                                     lex_ml_basic_body,
-                                     lex_ml_basic_string_close>;
-
-using lex_literal_char = exclude<either<in_range<0x00, 0x08>, in_range<0x0A, 0x1F>,
-                                        character<0x7F>, character<0x27>>>;
-using lex_apostrophe = character<'\''>;
-using lex_literal_string = sequence<lex_apostrophe,
-                                    repeat<lex_literal_char, unlimited>,
-                                    lex_apostrophe>;
-
-// the same reason as above.
-using lex_ml_literal_string_delim = repeat<lex_apostrophe, exactly<3>>;
-using lex_ml_literal_string_open  = lex_ml_literal_string_delim;
-using lex_ml_literal_string_close = sequence<
-        repeat<lex_apostrophe, exactly<3>>,
-        maybe<lex_apostrophe>, maybe<lex_apostrophe>
-    >;
-
-using lex_ml_literal_char = exclude<either<in_range<0x00, 0x08>,
-                                           in_range<0x0A, 0x1F>,
-                                           character<0x7F>,
-                                           lex_ml_literal_string_delim>>;
-using lex_ml_literal_body = repeat<either<lex_ml_literal_char, lex_newline>,
-                                   unlimited>;
-using lex_ml_literal_string = sequence<lex_ml_literal_string_open,
-                                       lex_ml_literal_body,
-                                       lex_ml_literal_string_close>;
-
-using lex_string = either<lex_ml_basic_string,   lex_basic_string,
-                          lex_ml_literal_string, lex_literal_string>;
-
-// ===========================================================================
-using lex_dot_sep = sequence<maybe<lex_ws>, character<'.'>, maybe<lex_ws>>;
-
-using lex_unquoted_key = repeat<either<lex_alpha, lex_digit,
-                                       character<'-'>, character<'_'>>,
-                                at_least<1>>;
-using lex_quoted_key = either<lex_basic_string, lex_literal_string>;
-using lex_simple_key = either<lex_unquoted_key, lex_quoted_key>;
-using lex_dotted_key = sequence<lex_simple_key,
-                                repeat<sequence<lex_dot_sep, lex_simple_key>,
-                                       at_least<1>
-                                       >
-                                >;
-using lex_key = either<lex_dotted_key, lex_simple_key>;
-
-using lex_keyval_sep = sequence<maybe<lex_ws>,
-                                character<'='>,
-                                maybe<lex_ws>>;
-
-using lex_std_table_open  = character<'['>;
-using lex_std_table_close = character<']'>;
-using lex_std_table       = sequence<lex_std_table_open,
-                                     maybe<lex_ws>,
-                                     lex_key,
-                                     maybe<lex_ws>,
-                                     lex_std_table_close>;
-
-using lex_array_table_open  = sequence<lex_std_table_open,  lex_std_table_open>;
-using lex_array_table_close = sequence<lex_std_table_close, lex_std_table_close>;
-using lex_array_table       = sequence<lex_array_table_open,
-                                       maybe<lex_ws>,
-                                       lex_key,
-                                       maybe<lex_ws>,
-                                       lex_array_table_close>;
-
-using lex_utf8_1byte = in_range<0x00, 0x7F>;
-using lex_utf8_2byte = sequence<
-        in_range<static_cast<char>(0xC2), static_cast<char>(0xDF)>,
-        in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>
-    >;
-using lex_utf8_3byte = sequence<either<
-        sequence<character<static_cast<char>(0xE0)>,                          in_range<static_cast<char>(0xA0), static_cast<char>(0xBF)>>,
-        sequence<in_range <static_cast<char>(0xE1), static_cast<char>(0xEC)>, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>,
-        sequence<character<static_cast<char>(0xED)>,                          in_range<static_cast<char>(0x80), static_cast<char>(0x9F)>>,
-        sequence<in_range <static_cast<char>(0xEE), static_cast<char>(0xEF)>, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>
-    >, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>;
-using lex_utf8_4byte = sequence<either<
-        sequence<character<static_cast<char>(0xF0)>,                          in_range<static_cast<char>(0x90), static_cast<char>(0xBF)>>,
-        sequence<in_range <static_cast<char>(0xF1), static_cast<char>(0xF3)>, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>,
-        sequence<character<static_cast<char>(0xF4)>,                          in_range<static_cast<char>(0x80), static_cast<char>(0x8F)>>
-    >, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>,
-       in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>;
-using lex_utf8_code = either<
-        lex_utf8_1byte,
-        lex_utf8_2byte,
-        lex_utf8_3byte,
-        lex_utf8_4byte
-    >;
-
-using lex_comment_start_symbol = character<'#'>;
-using lex_non_eol_ascii = either<character<0x09>, in_range<0x20, 0x7E>>;
-using lex_comment = sequence<lex_comment_start_symbol, repeat<either<
-    lex_non_eol_ascii, lex_utf8_2byte, lex_utf8_3byte, lex_utf8_4byte>, unlimited>>;
-
-} // detail
-} // toml
-#endif // TOML_LEXER_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/literal.hpp b/src/frontend/qt_sdl/toml/toml/literal.hpp
deleted file mode 100644
index 04fbbc13..00000000
--- a/src/frontend/qt_sdl/toml/toml/literal.hpp
+++ /dev/null
@@ -1,113 +0,0 @@
-//     Copyright Toru Niina 2019.
-// Distributed under the MIT License.
-#ifndef TOML11_LITERAL_HPP
-#define TOML11_LITERAL_HPP
-#include "parser.hpp"
-
-namespace toml
-{
-inline namespace literals
-{
-inline namespace toml_literals
-{
-
-// implementation
-inline ::toml::basic_value<TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>
-literal_internal_impl(::toml::detail::location loc)
-{
-    using value_type = ::toml::basic_value<
-        TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>;
-    // if there are some comments or empty lines, skip them.
-    using skip_line = ::toml::detail::repeat<toml::detail::sequence<
-            ::toml::detail::maybe<::toml::detail::lex_ws>,
-            ::toml::detail::maybe<::toml::detail::lex_comment>,
-            ::toml::detail::lex_newline
-        >, ::toml::detail::at_least<1>>;
-    skip_line::invoke(loc);
-
-    // if there are some whitespaces before a value, skip them.
-    using skip_ws = ::toml::detail::repeat<
-        ::toml::detail::lex_ws, ::toml::detail::at_least<1>>;
-    skip_ws::invoke(loc);
-
-    // to distinguish arrays and tables, first check it is a table or not.
-    //
-    // "[1,2,3]"_toml;   // this is an array
-    // "[table]"_toml;   // a table that has an empty table named "table" inside.
-    // "[[1,2,3]]"_toml; // this is an array of arrays
-    // "[[table]]"_toml; // this is a table that has an array of tables inside.
-    //
-    // "[[1]]"_toml;     // this can be both... (currently it becomes a table)
-    // "1 = [{}]"_toml;  // this is a table that has an array of table named 1.
-    // "[[1,]]"_toml;    // this is an array of arrays.
-    // "[[1],]"_toml;    // this also.
-
-    const auto the_front = loc.iter();
-
-    const bool is_table_key = ::toml::detail::lex_std_table::invoke(loc);
-    loc.reset(the_front);
-
-    const bool is_aots_key  = ::toml::detail::lex_array_table::invoke(loc);
-    loc.reset(the_front);
-
-    // If it is neither a table-key or a array-of-table-key, it may be a value.
-    if(!is_table_key && !is_aots_key)
-    {
-        if(auto data = ::toml::detail::parse_value<value_type>(loc))
-        {
-            return data.unwrap();
-        }
-    }
-
-    // Note that still it can be a table, because the literal might be something
-    // like the following.
-    // ```cpp
-    // R"( // c++11 raw string literals
-    //   key = "value"
-    //   int = 42
-    // )"_toml;
-    // ```
-    // It is a valid toml file.
-    // It should be parsed as if we parse a file with this content.
-
-    if(auto data = ::toml::detail::parse_toml_file<value_type>(loc))
-    {
-        return data.unwrap();
-    }
-    else // none of them.
-    {
-        throw ::toml::syntax_error(data.unwrap_err(), source_location(loc));
-    }
-
-}
-
-inline ::toml::basic_value<TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>
-operator"" _toml(const char* str, std::size_t len)
-{
-    ::toml::detail::location loc(
-            std::string("TOML literal encoded in a C++ code"),
-            std::vector<char>(str, str + len));
-    // literal length does not include the null character at the end.
-    return literal_internal_impl(std::move(loc));
-}
-
-// value of __cplusplus in C++2a/20 mode is not fixed yet along compilers.
-// So here we use the feature test macro for `char8_t` itself.
-#if defined(__cpp_char8_t) && __cpp_char8_t >= 201811L
-// value of u8"" literal has been changed from char to char8_t and char8_t is
-// NOT compatible to char
-inline ::toml::basic_value<TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>
-operator"" _toml(const char8_t* str, std::size_t len)
-{
-    ::toml::detail::location loc(
-            std::string("TOML literal encoded in a C++ code"),
-            std::vector<char>(reinterpret_cast<const char*>(str),
-                              reinterpret_cast<const char*>(str) + len));
-    return literal_internal_impl(std::move(loc));
-}
-#endif
-
-} // toml_literals
-} // literals
-} // toml
-#endif//TOML11_LITERAL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/parser.hpp b/src/frontend/qt_sdl/toml/toml/parser.hpp
deleted file mode 100644
index bfa5531f..00000000
--- a/src/frontend/qt_sdl/toml/toml/parser.hpp
+++ /dev/null
@@ -1,2416 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_PARSER_HPP
-#define TOML11_PARSER_HPP
-#include <cstring>
-#include <fstream>
-#include <sstream>
-
-#include "combinator.hpp"
-#include "lexer.hpp"
-#include "region.hpp"
-#include "result.hpp"
-#include "types.hpp"
-#include "value.hpp"
-
-#ifndef TOML11_DISABLE_STD_FILESYSTEM
-#ifdef __cpp_lib_filesystem
-#if __has_include(<filesystem>)
-#define TOML11_HAS_STD_FILESYSTEM
-#include <filesystem>
-#endif // has_include(<string_view>)
-#endif // __cpp_lib_filesystem
-#endif // TOML11_DISABLE_STD_FILESYSTEM
-
-namespace toml
-{
-namespace detail
-{
-
-inline result<std::pair<boolean, region>, std::string>
-parse_boolean(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_boolean::invoke(loc))
-    {
-        const auto reg = token.unwrap();
-        if     (reg.str() == "true")  {return ok(std::make_pair(true,  reg));}
-        else if(reg.str() == "false") {return ok(std::make_pair(false, reg));}
-        else // internal error.
-        {
-            throw internal_error(format_underline(
-                "toml::parse_boolean: internal error",
-                {{source_location(reg), "invalid token"}}),
-                source_location(reg));
-        }
-    }
-    loc.reset(first); //rollback
-    return err(format_underline("toml::parse_boolean: ",
-               {{source_location(loc), "the next token is not a boolean"}}));
-}
-
-inline result<std::pair<integer, region>, std::string>
-parse_binary_integer(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_bin_int::invoke(loc))
-    {
-        auto str = token.unwrap().str();
-        assert(str.size() > 2); // minimum -> 0b1
-        integer retval(0), base(1);
-        for(auto i(str.rbegin()), e(str.rend() - 2); i!=e; ++i)
-        {
-            if     (*i == '1'){retval += base; base *= 2;}
-            else if(*i == '0'){base *= 2;}
-            else if(*i == '_'){/* do nothing. */}
-            else // internal error.
-            {
-                throw internal_error(format_underline(
-                    "toml::parse_integer: internal error",
-                    {{source_location(token.unwrap()), "invalid token"}}),
-                    source_location(loc));
-            }
-        }
-        return ok(std::make_pair(retval, token.unwrap()));
-    }
-    loc.reset(first);
-    return err(format_underline("toml::parse_binary_integer:",
-               {{source_location(loc), "the next token is not an integer"}}));
-}
-
-inline result<std::pair<integer, region>, std::string>
-parse_octal_integer(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_oct_int::invoke(loc))
-    {
-        auto str = token.unwrap().str();
-        str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
-        str.erase(str.begin()); str.erase(str.begin()); // remove `0o` prefix
-
-        std::istringstream iss(str);
-        integer retval(0);
-        iss >> std::oct >> retval;
-        return ok(std::make_pair(retval, token.unwrap()));
-    }
-    loc.reset(first);
-    return err(format_underline("toml::parse_octal_integer:",
-               {{source_location(loc), "the next token is not an integer"}}));
-}
-
-inline result<std::pair<integer, region>, std::string>
-parse_hexadecimal_integer(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_hex_int::invoke(loc))
-    {
-        auto str = token.unwrap().str();
-        str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
-        str.erase(str.begin()); str.erase(str.begin()); // remove `0x` prefix
-
-        std::istringstream iss(str);
-        integer retval(0);
-        iss >> std::hex >> retval;
-        return ok(std::make_pair(retval, token.unwrap()));
-    }
-    loc.reset(first);
-    return err(format_underline("toml::parse_hexadecimal_integer",
-               {{source_location(loc), "the next token is not an integer"}}));
-}
-
-inline result<std::pair<integer, region>, std::string>
-parse_integer(location& loc)
-{
-    const auto first = loc.iter();
-    if(first != loc.end() && *first == '0')
-    {
-        const auto second = std::next(first);
-        if(second == loc.end()) // the token is just zero.
-        {
-            loc.advance();
-            return ok(std::make_pair(0, region(loc, first, second)));
-        }
-
-        if(*second == 'b') {return parse_binary_integer     (loc);} // 0b1100
-        if(*second == 'o') {return parse_octal_integer      (loc);} // 0o775
-        if(*second == 'x') {return parse_hexadecimal_integer(loc);} // 0xC0FFEE
-
-        if(std::isdigit(*second))
-        {
-            return err(format_underline("toml::parse_integer: "
-                "leading zero in an Integer is not allowed.",
-                {{source_location(loc), "leading zero"}}));
-        }
-        else if(std::isalpha(*second))
-        {
-             return err(format_underline("toml::parse_integer: "
-                "unknown integer prefix appeared.",
-                {{source_location(loc), "none of 0x, 0o, 0b"}}));
-        }
-    }
-
-    if(const auto token = lex_dec_int::invoke(loc))
-    {
-        auto str = token.unwrap().str();
-        str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
-
-        std::istringstream iss(str);
-        integer retval(0);
-        iss >> retval;
-        return ok(std::make_pair(retval, token.unwrap()));
-    }
-    loc.reset(first);
-    return err(format_underline("toml::parse_integer: ",
-               {{source_location(loc), "the next token is not an integer"}}));
-}
-
-inline result<std::pair<floating, region>, std::string>
-parse_floating(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_float::invoke(loc))
-    {
-        auto str = token.unwrap().str();
-        if(str == "inf" || str == "+inf")
-        {
-            if(std::numeric_limits<floating>::has_infinity)
-            {
-                return ok(std::make_pair(
-                    std::numeric_limits<floating>::infinity(), token.unwrap()));
-            }
-            else
-            {
-                throw std::domain_error("toml::parse_floating: inf value found"
-                    " but the current environment does not support inf. Please"
-                    " make sure that the floating-point implementation conforms"
-                    " IEEE 754/ISO 60559 international standard.");
-            }
-        }
-        else if(str == "-inf")
-        {
-            if(std::numeric_limits<floating>::has_infinity)
-            {
-                return ok(std::make_pair(
-                    -std::numeric_limits<floating>::infinity(), token.unwrap()));
-            }
-            else
-            {
-                throw std::domain_error("toml::parse_floating: inf value found"
-                    " but the current environment does not support inf. Please"
-                    " make sure that the floating-point implementation conforms"
-                    " IEEE 754/ISO 60559 international standard.");
-            }
-        }
-        else if(str == "nan" || str == "+nan")
-        {
-            if(std::numeric_limits<floating>::has_quiet_NaN)
-            {
-                return ok(std::make_pair(
-                    std::numeric_limits<floating>::quiet_NaN(), token.unwrap()));
-            }
-            else if(std::numeric_limits<floating>::has_signaling_NaN)
-            {
-                return ok(std::make_pair(
-                    std::numeric_limits<floating>::signaling_NaN(), token.unwrap()));
-            }
-            else
-            {
-                throw std::domain_error("toml::parse_floating: NaN value found"
-                    " but the current environment does not support NaN. Please"
-                    " make sure that the floating-point implementation conforms"
-                    " IEEE 754/ISO 60559 international standard.");
-            }
-        }
-        else if(str == "-nan")
-        {
-            if(std::numeric_limits<floating>::has_quiet_NaN)
-            {
-                return ok(std::make_pair(
-                    -std::numeric_limits<floating>::quiet_NaN(), token.unwrap()));
-            }
-            else if(std::numeric_limits<floating>::has_signaling_NaN)
-            {
-                return ok(std::make_pair(
-                    -std::numeric_limits<floating>::signaling_NaN(), token.unwrap()));
-            }
-            else
-            {
-                throw std::domain_error("toml::parse_floating: NaN value found"
-                    " but the current environment does not support NaN. Please"
-                    " make sure that the floating-point implementation conforms"
-                    " IEEE 754/ISO 60559 international standard.");
-            }
-        }
-        str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
-        std::istringstream iss(str);
-        floating v(0.0);
-        iss >> v;
-        return ok(std::make_pair(v, token.unwrap()));
-    }
-    loc.reset(first);
-    return err(format_underline("toml::parse_floating: ",
-               {{source_location(loc), "the next token is not a float"}}));
-}
-
-inline std::string read_utf8_codepoint(const region& reg, const location& loc)
-{
-    const auto str = reg.str().substr(1);
-    std::uint_least32_t codepoint;
-    std::istringstream iss(str);
-    iss >> std::hex >> codepoint;
-
-    const auto to_char = [](const std::uint_least32_t i) noexcept -> char {
-        const auto uc = static_cast<unsigned char>(i);
-        return *reinterpret_cast<const char*>(std::addressof(uc));
-    };
-
-    std::string character;
-    if(codepoint < 0x80) // U+0000 ... U+0079 ; just an ASCII.
-    {
-        character += static_cast<char>(codepoint);
-    }
-    else if(codepoint < 0x800) //U+0080 ... U+07FF
-    {
-        // 110yyyyx 10xxxxxx; 0x3f == 0b0011'1111
-        character += to_char(0xC0| codepoint >> 6);
-        character += to_char(0x80|(codepoint & 0x3F));
-    }
-    else if(codepoint < 0x10000) // U+0800...U+FFFF
-    {
-        if(0xD800 <= codepoint && codepoint <= 0xDFFF)
-        {
-            throw syntax_error(format_underline(
-                "toml::read_utf8_codepoint: codepoints in the range "
-                "[0xD800, 0xDFFF] are not valid UTF-8.", {{
-                    source_location(loc), "not a valid UTF-8 codepoint"
-                }}), source_location(loc));
-        }
-        assert(codepoint < 0xD800 || 0xDFFF < codepoint);
-        // 1110yyyy 10yxxxxx 10xxxxxx
-        character += to_char(0xE0| codepoint >> 12);
-        character += to_char(0x80|(codepoint >> 6 & 0x3F));
-        character += to_char(0x80|(codepoint      & 0x3F));
-    }
-    else if(codepoint < 0x110000) // U+010000 ... U+10FFFF
-    {
-        // 11110yyy 10yyxxxx 10xxxxxx 10xxxxxx
-        character += to_char(0xF0| codepoint >> 18);
-        character += to_char(0x80|(codepoint >> 12 & 0x3F));
-        character += to_char(0x80|(codepoint >> 6  & 0x3F));
-        character += to_char(0x80|(codepoint       & 0x3F));
-    }
-    else // out of UTF-8 region
-    {
-        throw syntax_error(format_underline("toml::read_utf8_codepoint:"
-            " input codepoint is too large.",
-            {{source_location(loc), "should be in [0x00..0x10FFFF]"}}),
-            source_location(loc));
-    }
-    return character;
-}
-
-inline result<std::string, std::string> parse_escape_sequence(location& loc)
-{
-    const auto first = loc.iter();
-    if(first == loc.end() || *first != '\\')
-    {
-        return err(format_underline("toml::parse_escape_sequence: ", {{
-            source_location(loc), "the next token is not a backslash \"\\\""}}));
-    }
-    loc.advance();
-    switch(*loc.iter())
-    {
-        case '\\':{loc.advance(); return ok(std::string("\\"));}
-        case '"' :{loc.advance(); return ok(std::string("\""));}
-        case 'b' :{loc.advance(); return ok(std::string("\b"));}
-        case 't' :{loc.advance(); return ok(std::string("\t"));}
-        case 'n' :{loc.advance(); return ok(std::string("\n"));}
-        case 'f' :{loc.advance(); return ok(std::string("\f"));}
-        case 'r' :{loc.advance(); return ok(std::string("\r"));}
-        case 'u' :
-        {
-            if(const auto token = lex_escape_unicode_short::invoke(loc))
-            {
-                return ok(read_utf8_codepoint(token.unwrap(), loc));
-            }
-            else
-            {
-                return err(format_underline("parse_escape_sequence: "
-                           "invalid token found in UTF-8 codepoint uXXXX.",
-                           {{source_location(loc), "here"}}));
-            }
-        }
-        case 'U':
-        {
-            if(const auto token = lex_escape_unicode_long::invoke(loc))
-            {
-                return ok(read_utf8_codepoint(token.unwrap(), loc));
-            }
-            else
-            {
-                return err(format_underline("parse_escape_sequence: "
-                           "invalid token found in UTF-8 codepoint Uxxxxxxxx",
-                           {{source_location(loc), "here"}}));
-            }
-        }
-    }
-
-    const auto msg = format_underline("parse_escape_sequence: "
-           "unknown escape sequence appeared.", {{source_location(loc),
-           "escape sequence is one of \\, \", b, t, n, f, r, uxxxx, Uxxxxxxxx"}},
-           /* Hints = */{"if you want to write backslash as just one backslash, "
-           "use literal string like: regex    = '<\\i\\c*\\s*>'"});
-    loc.reset(first);
-    return err(msg);
-}
-
-inline std::ptrdiff_t check_utf8_validity(const std::string& reg)
-{
-    location loc("tmp", reg);
-    const auto u8 = repeat<lex_utf8_code, unlimited>::invoke(loc);
-    if(!u8 || loc.iter() != loc.end())
-    {
-        const auto error_location = std::distance(loc.begin(), loc.iter());
-        assert(0 <= error_location);
-        return error_location;
-    }
-    return -1;
-}
-
-inline result<std::pair<toml::string, region>, std::string>
-parse_ml_basic_string(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_ml_basic_string::invoke(loc))
-    {
-        auto inner_loc = loc;
-        inner_loc.reset(first);
-
-        std::string retval;
-        retval.reserve(token.unwrap().size());
-
-        auto delim = lex_ml_basic_string_open::invoke(inner_loc);
-        if(!delim)
-        {
-            throw internal_error(format_underline(
-                "parse_ml_basic_string: invalid token",
-                {{source_location(inner_loc), "should be \"\"\""}}),
-                source_location(inner_loc));
-        }
-        // immediate newline is ignored (if exists)
-        /* discard return value */ lex_newline::invoke(inner_loc);
-
-        delim = none();
-        while(!delim)
-        {
-            using lex_unescaped_seq = repeat<
-                either<lex_ml_basic_unescaped, lex_newline>, unlimited>;
-            if(auto unescaped = lex_unescaped_seq::invoke(inner_loc))
-            {
-                retval += unescaped.unwrap().str();
-            }
-            if(auto escaped = parse_escape_sequence(inner_loc))
-            {
-                retval += escaped.unwrap();
-            }
-            if(auto esc_nl = lex_ml_basic_escaped_newline::invoke(inner_loc))
-            {
-                // ignore newline after escape until next non-ws char
-            }
-            if(inner_loc.iter() == inner_loc.end())
-            {
-                throw internal_error(format_underline(
-                    "parse_ml_basic_string: unexpected end of region",
-                    {{source_location(inner_loc), "not sufficient token"}}),
-                    source_location(inner_loc));
-            }
-            delim = lex_ml_basic_string_close::invoke(inner_loc);
-        }
-        // `lex_ml_basic_string_close` allows 3 to 5 `"`s to allow 1 or 2 `"`s
-        // at just before the delimiter. Here, we need to attach `"`s at the
-        // end of the string body, if it exists.
-        // For detail, see the definition of `lex_ml_basic_string_close`.
-        assert(std::all_of(delim.unwrap().first(), delim.unwrap().last(),
-                           [](const char c) noexcept {return c == '\"';}));
-        switch(delim.unwrap().size())
-        {
-            case 3: {break;}
-            case 4: {retval += "\"";  break;}
-            case 5: {retval += "\"\""; break;}
-            default:
-            {
-                throw internal_error(format_underline(
-                    "parse_ml_basic_string: closing delimiter has invalid length",
-                    {{source_location(inner_loc), "end of this"}}),
-                    source_location(inner_loc));
-            }
-        }
-
-        const auto err_loc = check_utf8_validity(token.unwrap().str());
-        if(err_loc == -1)
-        {
-            return ok(std::make_pair(toml::string(retval), token.unwrap()));
-        }
-        else
-        {
-            inner_loc.reset(first);
-            inner_loc.advance(err_loc);
-            throw syntax_error(format_underline(
-                "parse_ml_basic_string: invalid utf8 sequence found",
-                {{source_location(inner_loc), "here"}}),
-                source_location(inner_loc));
-        }
-    }
-    else
-    {
-        loc.reset(first);
-        return err(format_underline("toml::parse_ml_basic_string: "
-                   "the next token is not a valid multiline string",
-                   {{source_location(loc), "here"}}));
-    }
-}
-
-inline result<std::pair<toml::string, region>, std::string>
-parse_basic_string(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_basic_string::invoke(loc))
-    {
-        auto inner_loc = loc;
-        inner_loc.reset(first);
-
-        auto quot = lex_quotation_mark::invoke(inner_loc);
-        if(!quot)
-        {
-            throw internal_error(format_underline("parse_basic_string: "
-                "invalid token", {{source_location(inner_loc), "should be \""}}),
-                source_location(inner_loc));
-        }
-
-        std::string retval;
-        retval.reserve(token.unwrap().size());
-
-        quot = none();
-        while(!quot)
-        {
-            using lex_unescaped_seq = repeat<lex_basic_unescaped, unlimited>;
-            if(auto unescaped = lex_unescaped_seq::invoke(inner_loc))
-            {
-                retval += unescaped.unwrap().str();
-            }
-            if(auto escaped = parse_escape_sequence(inner_loc))
-            {
-                retval += escaped.unwrap();
-            }
-            if(inner_loc.iter() == inner_loc.end())
-            {
-                throw internal_error(format_underline(
-                    "parse_basic_string: unexpected end of region",
-                    {{source_location(inner_loc), "not sufficient token"}}),
-                    source_location(inner_loc));
-            }
-            quot = lex_quotation_mark::invoke(inner_loc);
-        }
-
-        const auto err_loc = check_utf8_validity(token.unwrap().str());
-        if(err_loc == -1)
-        {
-            return ok(std::make_pair(toml::string(retval), token.unwrap()));
-        }
-        else
-        {
-            inner_loc.reset(first);
-            inner_loc.advance(err_loc);
-            throw syntax_error(format_underline(
-                "parse_ml_basic_string: invalid utf8 sequence found",
-                {{source_location(inner_loc), "here"}}),
-                source_location(inner_loc));
-        }
-    }
-    else
-    {
-        loc.reset(first); // rollback
-        return err(format_underline("toml::parse_basic_string: "
-                   "the next token is not a valid string",
-                   {{source_location(loc), "here"}}));
-    }
-}
-
-inline result<std::pair<toml::string, region>, std::string>
-parse_ml_literal_string(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_ml_literal_string::invoke(loc))
-    {
-        location inner_loc(loc.name(), token.unwrap().str());
-
-        const auto open = lex_ml_literal_string_open::invoke(inner_loc);
-        if(!open)
-        {
-            throw internal_error(format_underline(
-                "parse_ml_literal_string: invalid token",
-                {{source_location(inner_loc), "should be '''"}}),
-                source_location(inner_loc));
-        }
-        // immediate newline is ignored (if exists)
-        /* discard return value */ lex_newline::invoke(inner_loc);
-
-        const auto body = lex_ml_literal_body::invoke(inner_loc);
-
-        const auto close = lex_ml_literal_string_close::invoke(inner_loc);
-        if(!close)
-        {
-            throw internal_error(format_underline(
-                "parse_ml_literal_string: invalid token",
-                {{source_location(inner_loc), "should be '''"}}),
-                source_location(inner_loc));
-        }
-        // `lex_ml_literal_string_close` allows 3 to 5 `'`s to allow 1 or 2 `'`s
-        // at just before the delimiter. Here, we need to attach `'`s at the
-        // end of the string body, if it exists.
-        // For detail, see the definition of `lex_ml_basic_string_close`.
-
-        std::string retval = body.unwrap().str();
-        assert(std::all_of(close.unwrap().first(), close.unwrap().last(),
-                           [](const char c) noexcept {return c == '\'';}));
-        switch(close.unwrap().size())
-        {
-            case 3: {break;}
-            case 4: {retval += "'";  break;}
-            case 5: {retval += "''"; break;}
-            default:
-            {
-                throw internal_error(format_underline(
-                    "parse_ml_literal_string: closing delimiter has invalid length",
-                    {{source_location(inner_loc), "end of this"}}),
-                    source_location(inner_loc));
-            }
-        }
-
-        const auto err_loc = check_utf8_validity(token.unwrap().str());
-        if(err_loc == -1)
-        {
-            return ok(std::make_pair(toml::string(retval, toml::string_t::literal),
-                                     token.unwrap()));
-        }
-        else
-        {
-            inner_loc.reset(first);
-            inner_loc.advance(err_loc);
-            throw syntax_error(format_underline(
-                "parse_ml_basic_string: invalid utf8 sequence found",
-                {{source_location(inner_loc), "here"}}),
-                source_location(inner_loc));
-        }
-    }
-    else
-    {
-        loc.reset(first); // rollback
-        return err(format_underline("toml::parse_ml_literal_string: "
-                   "the next token is not a valid multiline literal string",
-                   {{source_location(loc), "here"}}));
-    }
-}
-
-inline result<std::pair<toml::string, region>, std::string>
-parse_literal_string(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_literal_string::invoke(loc))
-    {
-        location inner_loc(loc.name(), token.unwrap().str());
-
-        const auto open = lex_apostrophe::invoke(inner_loc);
-        if(!open)
-        {
-            throw internal_error(format_underline(
-                "parse_literal_string: invalid token",
-                {{source_location(inner_loc), "should be '"}}),
-                source_location(inner_loc));
-        }
-
-        const auto body = repeat<lex_literal_char, unlimited>::invoke(inner_loc);
-
-        const auto close = lex_apostrophe::invoke(inner_loc);
-        if(!close)
-        {
-            throw internal_error(format_underline(
-                "parse_literal_string: invalid token",
-                {{source_location(inner_loc), "should be '"}}),
-                source_location(inner_loc));
-        }
-
-        const auto err_loc = check_utf8_validity(token.unwrap().str());
-        if(err_loc == -1)
-        {
-            return ok(std::make_pair(
-                      toml::string(body.unwrap().str(), toml::string_t::literal),
-                      token.unwrap()));
-        }
-        else
-        {
-            inner_loc.reset(first);
-            inner_loc.advance(err_loc);
-            throw syntax_error(format_underline(
-                "parse_ml_basic_string: invalid utf8 sequence found",
-                {{source_location(inner_loc), "here"}}),
-                source_location(inner_loc));
-        }
-    }
-    else
-    {
-        loc.reset(first); // rollback
-        return err(format_underline("toml::parse_literal_string: "
-                   "the next token is not a valid literal string",
-                   {{source_location(loc), "here"}}));
-    }
-}
-
-inline result<std::pair<toml::string, region>, std::string>
-parse_string(location& loc)
-{
-    if(loc.iter() != loc.end() && *(loc.iter()) == '"')
-    {
-        if(loc.iter() + 1 != loc.end() && *(loc.iter() + 1) == '"' &&
-           loc.iter() + 2 != loc.end() && *(loc.iter() + 2) == '"')
-        {
-            return parse_ml_basic_string(loc);
-        }
-        else
-        {
-            return parse_basic_string(loc);
-        }
-    }
-    else if(loc.iter() != loc.end() && *(loc.iter()) == '\'')
-    {
-        if(loc.iter() + 1 != loc.end() && *(loc.iter() + 1) == '\'' &&
-           loc.iter() + 2 != loc.end() && *(loc.iter() + 2) == '\'')
-        {
-            return parse_ml_literal_string(loc);
-        }
-        else
-        {
-            return parse_literal_string(loc);
-        }
-    }
-    return err(format_underline("toml::parse_string: ",
-                {{source_location(loc), "the next token is not a string"}}));
-}
-
-inline result<std::pair<local_date, region>, std::string>
-parse_local_date(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_local_date::invoke(loc))
-    {
-        location inner_loc(loc.name(), token.unwrap().str());
-
-        const auto y = lex_date_fullyear::invoke(inner_loc);
-        if(!y || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != '-')
-        {
-            throw internal_error(format_underline(
-                "toml::parse_inner_local_date: invalid year format",
-                {{source_location(inner_loc), "should be `-`"}}),
-                source_location(inner_loc));
-        }
-        inner_loc.advance();
-        const auto m = lex_date_month::invoke(inner_loc);
-        if(!m || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != '-')
-        {
-            throw internal_error(format_underline(
-                "toml::parse_local_date: invalid month format",
-                {{source_location(inner_loc), "should be `-`"}}),
-                source_location(inner_loc));
-        }
-        inner_loc.advance();
-        const auto d = lex_date_mday::invoke(inner_loc);
-        if(!d)
-        {
-            throw internal_error(format_underline(
-                "toml::parse_local_date: invalid day format",
-                {{source_location(inner_loc), "here"}}),
-                source_location(inner_loc));
-        }
-
-        const auto year  = static_cast<std::int16_t>(from_string<int>(y.unwrap().str(), 0));
-        const auto month = static_cast<std::int8_t >(from_string<int>(m.unwrap().str(), 0));
-        const auto day   = static_cast<std::int8_t >(from_string<int>(d.unwrap().str(), 0));
-
-        // We briefly check whether the input date is valid or not. But here, we
-        // only check if the RFC3339 compliance.
-        //     Actually there are several special date that does not exist,
-        // because of historical reasons, such as 1582/10/5-1582/10/14 (only in
-        // several countries). But here, we do not care about such a complicated
-        // rule. It makes the code complicated and there is only low probability
-        // that such a specific date is needed in practice. If someone need to
-        // validate date accurately, that means that the one need a specialized
-        // library for their purpose in a different layer.
-        {
-            const bool is_leap = (year % 4 == 0) && ((year % 100 != 0) || (year % 400 == 0));
-            const auto max_day = (month == 2) ? (is_leap ? 29 : 28) :
-                ((month == 4 || month == 6 || month == 9 || month == 11) ? 30 : 31);
-
-            if((month < 1 || 12 < month) || (day < 1 || max_day < day))
-            {
-                throw syntax_error(format_underline("toml::parse_date: "
-                    "invalid date: it does not conform RFC3339.", {{
-                    source_location(loc), "month should be 01-12, day should be"
-                    " 01-28,29,30,31, depending on month/year."
-                    }}), source_location(inner_loc));
-            }
-        }
-        return ok(std::make_pair(local_date(year, static_cast<month_t>(month - 1), day),
-                                 token.unwrap()));
-    }
-    else
-    {
-        loc.reset(first);
-        return err(format_underline("toml::parse_local_date: ",
-            {{source_location(loc), "the next token is not a local_date"}}));
-    }
-}
-
-inline result<std::pair<local_time, region>, std::string>
-parse_local_time(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_local_time::invoke(loc))
-    {
-        location inner_loc(loc.name(), token.unwrap().str());
-
-        const auto h = lex_time_hour::invoke(inner_loc);
-        if(!h || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != ':')
-        {
-            throw internal_error(format_underline(
-                "toml::parse_local_time: invalid year format",
-                {{source_location(inner_loc), "should be `:`"}}),
-                source_location(inner_loc));
-        }
-        inner_loc.advance();
-        const auto m = lex_time_minute::invoke(inner_loc);
-        if(!m || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != ':')
-        {
-            throw internal_error(format_underline(
-                "toml::parse_local_time: invalid month format",
-                {{source_location(inner_loc), "should be `:`"}}),
-                source_location(inner_loc));
-        }
-        inner_loc.advance();
-        const auto s = lex_time_second::invoke(inner_loc);
-        if(!s)
-        {
-            throw internal_error(format_underline(
-                "toml::parse_local_time: invalid second format",
-                {{source_location(inner_loc), "here"}}),
-                source_location(inner_loc));
-        }
-
-        const int hour   = from_string<int>(h.unwrap().str(), 0);
-        const int minute = from_string<int>(m.unwrap().str(), 0);
-        const int second = from_string<int>(s.unwrap().str(), 0);
-
-        if((hour   < 0 || 23 < hour) || (minute < 0 || 59 < minute) ||
-           (second < 0 || 60 < second)) // it may be leap second
-        {
-            throw syntax_error(format_underline("toml::parse_time: "
-                "invalid time: it does not conform RFC3339.", {{
-                source_location(loc), "hour should be 00-23, minute should be"
-                " 00-59, second should be 00-60 (depending on the leap"
-                " second rules.)"}}), source_location(inner_loc));
-        }
-
-        local_time time(hour, minute, second, 0, 0);
-
-        const auto before_secfrac = inner_loc.iter();
-        if(const auto secfrac = lex_time_secfrac::invoke(inner_loc))
-        {
-            auto sf = secfrac.unwrap().str();
-            sf.erase(sf.begin()); // sf.front() == '.'
-            switch(sf.size() % 3)
-            {
-                case 2:  sf += '0';  break;
-                case 1:  sf += "00"; break;
-                case 0:  break;
-                default: break;
-            }
-            if(sf.size() >= 9)
-            {
-                time.millisecond = from_string<std::uint16_t>(sf.substr(0, 3), 0u);
-                time.microsecond = from_string<std::uint16_t>(sf.substr(3, 3), 0u);
-                time.nanosecond  = from_string<std::uint16_t>(sf.substr(6, 3), 0u);
-            }
-            else if(sf.size() >= 6)
-            {
-                time.millisecond = from_string<std::uint16_t>(sf.substr(0, 3), 0u);
-                time.microsecond = from_string<std::uint16_t>(sf.substr(3, 3), 0u);
-            }
-            else if(sf.size() >= 3)
-            {
-                time.millisecond = from_string<std::uint16_t>(sf, 0u);
-                time.microsecond = 0u;
-            }
-        }
-        else
-        {
-            if(before_secfrac != inner_loc.iter())
-            {
-                throw internal_error(format_underline(
-                    "toml::parse_local_time: invalid subsecond format",
-                    {{source_location(inner_loc), "here"}}),
-                source_location(inner_loc));
-            }
-        }
-        return ok(std::make_pair(time, token.unwrap()));
-    }
-    else
-    {
-        loc.reset(first);
-        return err(format_underline("toml::parse_local_time: ",
-            {{source_location(loc), "the next token is not a local_time"}}));
-    }
-}
-
-inline result<std::pair<local_datetime, region>, std::string>
-parse_local_datetime(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_local_date_time::invoke(loc))
-    {
-        location inner_loc(loc.name(), token.unwrap().str());
-        const auto date = parse_local_date(inner_loc);
-        if(!date || inner_loc.iter() == inner_loc.end())
-        {
-            throw internal_error(format_underline(
-                "toml::parse_local_datetime: invalid datetime format",
-                {{source_location(inner_loc), "date, not datetime"}}),
-                source_location(inner_loc));
-        }
-        const char delim = *(inner_loc.iter());
-        if(delim != 'T' && delim != 't' && delim != ' ')
-        {
-            throw internal_error(format_underline(
-                "toml::parse_local_datetime: invalid datetime format",
-                {{source_location(inner_loc), "should be `T` or ` ` (space)"}}),
-                source_location(inner_loc));
-        }
-        inner_loc.advance();
-        const auto time = parse_local_time(inner_loc);
-        if(!time)
-        {
-            throw internal_error(format_underline(
-                "toml::parse_local_datetime: invalid datetime format",
-                {{source_location(inner_loc), "invalid time format"}}),
-                source_location(inner_loc));
-        }
-        return ok(std::make_pair(
-            local_datetime(date.unwrap().first, time.unwrap().first),
-            token.unwrap()));
-    }
-    else
-    {
-        loc.reset(first);
-        return err(format_underline("toml::parse_local_datetime: ",
-            {{source_location(loc), "the next token is not a local_datetime"}}));
-    }
-}
-
-inline result<std::pair<offset_datetime, region>, std::string>
-parse_offset_datetime(location& loc)
-{
-    const auto first = loc.iter();
-    if(const auto token = lex_offset_date_time::invoke(loc))
-    {
-        location inner_loc(loc.name(), token.unwrap().str());
-        const auto datetime = parse_local_datetime(inner_loc);
-        if(!datetime || inner_loc.iter() == inner_loc.end())
-        {
-            throw internal_error(format_underline(
-                "toml::parse_offset_datetime: invalid datetime format",
-                {{source_location(inner_loc), "date, not datetime"}}),
-                source_location(inner_loc));
-        }
-        time_offset offset(0, 0);
-        if(const auto ofs = lex_time_numoffset::invoke(inner_loc))
-        {
-            const auto str = ofs.unwrap().str();
-
-            const auto hour   = from_string<int>(str.substr(1,2), 0);
-            const auto minute = from_string<int>(str.substr(4,2), 0);
-
-            if((hour < 0 || 23 < hour) || (minute < 0 || 59 < minute))
-            {
-                throw syntax_error(format_underline("toml::parse_offset_datetime: "
-                    "invalid offset: it does not conform RFC3339.", {{
-                    source_location(loc), "month should be 01-12, day should be"
-                    " 01-28,29,30,31, depending on month/year."
-                    }}), source_location(inner_loc));
-            }
-
-            if(str.front() == '+')
-            {
-                offset = time_offset(hour, minute);
-            }
-            else
-            {
-                offset = time_offset(-hour, -minute);
-            }
-        }
-        else if(*inner_loc.iter() != 'Z' && *inner_loc.iter() != 'z')
-        {
-            throw internal_error(format_underline(
-                "toml::parse_offset_datetime: invalid datetime format",
-                {{source_location(inner_loc), "should be `Z` or `+HH:MM`"}}),
-                source_location(inner_loc));
-        }
-        return ok(std::make_pair(offset_datetime(datetime.unwrap().first, offset),
-                                 token.unwrap()));
-    }
-    else
-    {
-        loc.reset(first);
-        return err(format_underline("toml::parse_offset_datetime: ",
-            {{source_location(loc), "the next token is not a offset_datetime"}}));
-    }
-}
-
-inline result<std::pair<key, region>, std::string>
-parse_simple_key(location& loc)
-{
-    if(const auto bstr = parse_basic_string(loc))
-    {
-        return ok(std::make_pair(bstr.unwrap().first.str, bstr.unwrap().second));
-    }
-    if(const auto lstr = parse_literal_string(loc))
-    {
-        return ok(std::make_pair(lstr.unwrap().first.str, lstr.unwrap().second));
-    }
-    if(const auto bare = lex_unquoted_key::invoke(loc))
-    {
-        const auto reg = bare.unwrap();
-        return ok(std::make_pair(reg.str(), reg));
-    }
-    return err(format_underline("toml::parse_simple_key: ",
-            {{source_location(loc), "the next token is not a simple key"}}));
-}
-
-// dotted key become vector of keys
-inline result<std::pair<std::vector<key>, region>, std::string>
-parse_key(location& loc)
-{
-    const auto first = loc.iter();
-    // dotted key -> `foo.bar.baz` where several single keys are chained by
-    // dots. Whitespaces between keys and dots are allowed.
-    if(const auto token = lex_dotted_key::invoke(loc))
-    {
-        const auto reg = token.unwrap();
-        location inner_loc(loc.name(), reg.str());
-        std::vector<key> keys;
-
-        while(inner_loc.iter() != inner_loc.end())
-        {
-            lex_ws::invoke(inner_loc);
-            if(const auto k = parse_simple_key(inner_loc))
-            {
-                keys.push_back(k.unwrap().first);
-            }
-            else
-            {
-                throw internal_error(format_underline(
-                    "toml::detail::parse_key: dotted key contains invalid key",
-                    {{source_location(inner_loc), k.unwrap_err()}}),
-                    source_location(inner_loc));
-            }
-
-            lex_ws::invoke(inner_loc);
-            if(inner_loc.iter() == inner_loc.end())
-            {
-                break;
-            }
-            else if(*inner_loc.iter() == '.')
-            {
-                inner_loc.advance(); // to skip `.`
-            }
-            else
-            {
-                throw internal_error(format_underline("toml::parse_key: "
-                    "dotted key contains invalid key ",
-                    {{source_location(inner_loc), "should be `.`"}}),
-                    source_location(inner_loc));
-            }
-        }
-        return ok(std::make_pair(keys, reg));
-    }
-    loc.reset(first);
-
-    // simple_key: a single (basic_string|literal_string|bare key)
-    if(const auto smpl = parse_simple_key(loc))
-    {
-        return ok(std::make_pair(std::vector<key>(1, smpl.unwrap().first),
-                                 smpl.unwrap().second));
-    }
-    return err(format_underline("toml::parse_key: an invalid key appeared.",
-                {{source_location(loc), "is not a valid key"}}, {
-                "bare keys  : non-empty strings composed only of [A-Za-z0-9_-].",
-                "quoted keys: same as \"basic strings\" or 'literal strings'.",
-                "dotted keys: sequence of bare or quoted keys joined with a dot."
-                }));
-}
-
-// forward-decl to implement parse_array and parse_table
-template<typename Value>
-result<Value, std::string> parse_value(location&);
-
-template<typename Value>
-result<std::pair<typename Value::array_type, region>, std::string>
-parse_array(location& loc)
-{
-    using value_type = Value;
-    using array_type = typename value_type::array_type;
-
-    const auto first = loc.iter();
-    if(loc.iter() == loc.end())
-    {
-        return err("toml::parse_array: input is empty");
-    }
-    if(*loc.iter() != '[')
-    {
-        return err("toml::parse_array: token is not an array");
-    }
-    loc.advance();
-
-    using lex_ws_comment_newline = repeat<
-        either<lex_wschar, lex_newline, lex_comment>, unlimited>;
-
-    array_type retval;
-    while(loc.iter() != loc.end())
-    {
-        lex_ws_comment_newline::invoke(loc); // skip
-
-        if(loc.iter() != loc.end() && *loc.iter() == ']')
-        {
-            loc.advance(); // skip ']'
-            return ok(std::make_pair(retval,
-                      region(loc, first, loc.iter())));
-        }
-
-        if(auto val = parse_value<value_type>(loc))
-        {
-            // After TOML v1.0.0-rc.1, array becomes to be able to have values
-            // with different types. So here we will omit this by default.
-            //
-            // But some of the test-suite checks if the parser accepts a hetero-
-            // geneous arrays, so we keep this for a while.
-#ifdef TOML11_DISALLOW_HETEROGENEOUS_ARRAYS
-            if(!retval.empty() && retval.front().type() != val.as_ok().type())
-            {
-                auto array_start_loc = loc;
-                array_start_loc.reset(first);
-
-                throw syntax_error(format_underline("toml::parse_array: "
-                    "type of elements should be the same each other.", {
-                        {source_location(array_start_loc), "array starts here"},
-                        {
-                            retval.front().location(),
-                            "value has type " + stringize(retval.front().type())
-                        },
-                        {
-                            val.unwrap().location(),
-                            "value has different type, " + stringize(val.unwrap().type())
-                        }
-                    }), source_location(loc));
-            }
-#endif
-            retval.push_back(std::move(val.unwrap()));
-        }
-        else
-        {
-            auto array_start_loc = loc;
-            array_start_loc.reset(first);
-
-            throw syntax_error(format_underline("toml::parse_array: "
-                "value having invalid format appeared in an array", {
-                    {source_location(array_start_loc), "array starts here"},
-                    {source_location(loc), "it is not a valid value."}
-                }), source_location(loc));
-        }
-
-        using lex_array_separator = sequence<maybe<lex_ws_comment_newline>, character<','>>;
-        const auto sp = lex_array_separator::invoke(loc);
-        if(!sp)
-        {
-            lex_ws_comment_newline::invoke(loc);
-            if(loc.iter() != loc.end() && *loc.iter() == ']')
-            {
-                loc.advance(); // skip ']'
-                return ok(std::make_pair(retval,
-                          region(loc, first, loc.iter())));
-            }
-            else
-            {
-                auto array_start_loc = loc;
-                array_start_loc.reset(first);
-
-                throw syntax_error(format_underline("toml::parse_array:"
-                    " missing array separator `,` after a value", {
-                        {source_location(array_start_loc), "array starts here"},
-                        {source_location(loc),             "should be `,`"}
-                    }), source_location(loc));
-            }
-        }
-    }
-    loc.reset(first);
-    throw syntax_error(format_underline("toml::parse_array: "
-            "array did not closed by `]`",
-            {{source_location(loc), "should be closed"}}),
-            source_location(loc));
-}
-
-template<typename Value>
-result<std::pair<std::pair<std::vector<key>, region>, Value>, std::string>
-parse_key_value_pair(location& loc)
-{
-    using value_type = Value;
-
-    const auto first = loc.iter();
-    auto key_reg = parse_key(loc);
-    if(!key_reg)
-    {
-        std::string msg = std::move(key_reg.unwrap_err());
-        // if the next token is keyvalue-separator, it means that there are no
-        // key. then we need to show error as "empty key is not allowed".
-        if(const auto keyval_sep = lex_keyval_sep::invoke(loc))
-        {
-            loc.reset(first);
-            msg = format_underline("toml::parse_key_value_pair: "
-                "empty key is not allowed.",
-                {{source_location(loc), "key expected before '='"}});
-        }
-        return err(std::move(msg));
-    }
-
-    const auto kvsp = lex_keyval_sep::invoke(loc);
-    if(!kvsp)
-    {
-        std::string msg;
-        // if the line contains '=' after the invalid sequence, possibly the
-        // error is in the key (like, invalid character in bare key).
-        const auto line_end = std::find(loc.iter(), loc.end(), '\n');
-        if(std::find(loc.iter(), line_end, '=') != line_end)
-        {
-            msg = format_underline("toml::parse_key_value_pair: "
-                "invalid format for key",
-                {{source_location(loc), "invalid character in key"}},
-                {"Did you forget '.' to separate dotted-key?",
-                "Allowed characters for bare key are [0-9a-zA-Z_-]."});
-        }
-        else // if not, the error is lack of key-value separator.
-        {
-            msg = format_underline("toml::parse_key_value_pair: "
-                "missing key-value separator `=`",
-                {{source_location(loc), "should be `=`"}});
-        }
-        loc.reset(first);
-        return err(std::move(msg));
-    }
-
-    const auto after_kvsp = loc.iter(); // err msg
-    auto val = parse_value<value_type>(loc);
-    if(!val)
-    {
-        std::string msg;
-        loc.reset(after_kvsp);
-        // check there is something not a comment/whitespace after `=`
-        if(sequence<maybe<lex_ws>, maybe<lex_comment>, lex_newline>::invoke(loc))
-        {
-            loc.reset(after_kvsp);
-            msg = format_underline("toml::parse_key_value_pair: "
-                    "missing value after key-value separator '='",
-                    {{source_location(loc), "expected value, but got nothing"}});
-        }
-        else // there is something not a comment/whitespace, so invalid format.
-        {
-            msg = std::move(val.unwrap_err());
-        }
-        loc.reset(first);
-        return err(msg);
-    }
-    return ok(std::make_pair(std::move(key_reg.unwrap()),
-                             std::move(val.unwrap())));
-}
-
-// for error messages.
-template<typename InputIterator>
-std::string format_dotted_keys(InputIterator first, const InputIterator last)
-{
-    static_assert(std::is_same<key,
-        typename std::iterator_traits<InputIterator>::value_type>::value,"");
-
-    std::string retval(*first++);
-    for(; first != last; ++first)
-    {
-        retval += '.';
-        retval += *first;
-    }
-    return retval;
-}
-
-// forward decl for is_valid_forward_table_definition
-result<std::pair<std::vector<key>, region>, std::string>
-parse_table_key(location& loc);
-template<typename Value>
-result<std::pair<typename Value::table_type, region>, std::string>
-parse_inline_table(location& loc);
-
-// The following toml file is allowed.
-// ```toml
-// [a.b.c]     # here, table `a` has element `b`.
-// foo = "bar"
-// [a]         # merge a = {baz = "qux"} to a = {b = {...}}
-// baz = "qux"
-// ```
-// But the following is not allowed.
-// ```toml
-// [a]
-// b.c.foo = "bar"
-// [a]             # error! the same table [a] defined!
-// baz = "qux"
-// ```
-// The following is neither allowed.
-// ```toml
-// a = { b.c.foo = "bar"}
-// [a]             # error! the same table [a] defined!
-// baz = "qux"
-// ```
-// Here, it parses region of `tab->at(k)` as a table key and check the depth
-// of the key. If the key region points deeper node, it would be allowed.
-// Otherwise, the key points the same node. It would be rejected.
-template<typename Value, typename Iterator>
-bool is_valid_forward_table_definition(const Value& fwd, const Value& inserting,
-        Iterator key_first, Iterator key_curr, Iterator key_last)
-{
-    // ------------------------------------------------------------------------
-    // check type of the value to be inserted/merged
-
-    std::string inserting_reg = "";
-    if(const auto ptr = detail::get_region(inserting))
-    {
-        inserting_reg = ptr->str();
-    }
-    location inserting_def("internal", std::move(inserting_reg));
-    if(const auto inlinetable = parse_inline_table<Value>(inserting_def))
-    {
-        // check if we are overwriting existing table.
-        // ```toml
-        // # NG
-        // a.b = 42
-        // a = {d = 3.14}
-        // ```
-        // Inserting an inline table to a existing super-table is not allowed in
-        // any case. If we found it, we can reject it without further checking.
-        return false;
-    }
-
-    // Valid and invalid cases when inserting to the [a.b] table:
-    //
-    // ## Invalid
-    //
-    // ```toml
-    // # invalid
-    // [a]
-    // b.c.d = "foo"
-    // [a.b]       # a.b is already defined and closed
-    // d = "bar"
-    // ```
-    // ```toml
-    // # invalid
-    // a = {b.c.d = "foo"}
-    // [a.b] # a is already defined and inline table is closed
-    // d = "bar"
-    // ```
-    // ```toml
-    // # invalid
-    // a.b.c.d = "foo"
-    // [a.b] # a.b is already defined and dotted-key table is closed
-    // d = "bar"
-    // ```
-    //
-    // ## Valid
-    //
-    // ```toml
-    // # OK. a.b is defined, but is *overwritable*
-    // [a.b.c]
-    // d = "foo"
-    // [a.b]
-    // d = "bar"
-    // ```
-    // ```toml
-    // # OK. a.b is defined, but is *overwritable*
-    // [a]
-    // b.c.d = "foo"
-    // b.e = "bar"
-    // ```
-
-    // ------------------------------------------------------------------------
-    // check table defined before
-
-    std::string internal = "";
-    if(const auto ptr = detail::get_region(fwd))
-    {
-        internal = ptr->str();
-    }
-    location def("internal", std::move(internal));
-    if(const auto tabkeys = parse_table_key(def)) // [table.key]
-    {
-        // table keys always contains all the nodes from the root.
-        const auto& tks = tabkeys.unwrap().first;
-        if(std::size_t(std::distance(key_first, key_last)) == tks.size() &&
-           std::equal(tks.begin(), tks.end(), key_first))
-        {
-            // the keys are equivalent. it is not allowed.
-            return false;
-        }
-        // the keys are not equivalent. it is allowed.
-        return true;
-    }
-    if(const auto dotkeys = parse_key(def)) // a.b.c = "foo"
-    {
-        // consider the following case.
-        // [a]
-        // b.c = {d = 42}
-        // [a.b.c]
-        // e = 2.71
-        // this defines the table [a.b.c] twice. no?
-        if(const auto reopening_dotkey_by_table = parse_table_key(inserting_def))
-        {
-            // re-opening a dotkey-defined table by a table is invalid.
-            // only dotkey can append a key-val. Like:
-            // ```toml
-            // a.b.c = "foo"
-            // a.b.d = "bar" # OK. reopen `a.b` by dotkey
-            // [a.b]
-            // e = "bar" # Invalid. re-opening `a.b` by [a.b] is not allowed.
-            // ```
-            return false;
-        }
-
-        // a dotted key starts from the node representing a table in which the
-        // dotted key belongs to.
-        const auto& dks = dotkeys.unwrap().first;
-        if(std::size_t(std::distance(key_curr, key_last)) == dks.size() &&
-           std::equal(dks.begin(), dks.end(), key_curr))
-        {
-            // the keys are equivalent. it is not allowed.
-            return false;
-        }
-        // the keys are not equivalent. it is allowed.
-        return true;
-    }
-    return false;
-}
-
-template<typename Value, typename InputIterator>
-result<bool, std::string>
-insert_nested_key(typename Value::table_type& root, const Value& v,
-                  InputIterator iter, const InputIterator last,
-                  region key_reg,
-                  const bool is_array_of_table = false)
-{
-    static_assert(std::is_same<key,
-        typename std::iterator_traits<InputIterator>::value_type>::value,"");
-
-    using value_type = Value;
-    using table_type = typename value_type::table_type;
-    using array_type = typename value_type::array_type;
-
-    const auto first = iter;
-    assert(iter != last);
-
-    table_type* tab = std::addressof(root);
-    for(; iter != last; ++iter) // search recursively
-    {
-        const key& k = *iter;
-        if(std::next(iter) == last) // k is the last key
-        {
-            // XXX if the value is array-of-tables, there can be several
-            //     tables that are in the same array. in that case, we need to
-            //     find the last element and insert it to there.
-            if(is_array_of_table)
-            {
-                if(tab->count(k) == 1) // there is already an array of table
-                {
-                    if(tab->at(k).is_table())
-                    {
-                        // show special err msg for conflicting table
-                        throw syntax_error(format_underline(concat_to_string(
-                            "toml::insert_value: array of table (\"",
-                            format_dotted_keys(first, last),
-                            "\") cannot be defined"), {
-                                {tab->at(k).location(), "table already defined"},
-                                {v.location(), "this conflicts with the previous table"}
-                            }), v.location());
-                    }
-                    else if(!(tab->at(k).is_array()))
-                    {
-                        throw syntax_error(format_underline(concat_to_string(
-                            "toml::insert_value: array of table (\"",
-                            format_dotted_keys(first, last), "\") collides with"
-                            " existing value"), {
-                                {tab->at(k).location(),
-                                 concat_to_string("this ", tab->at(k).type(),
-                                                  " value already exists")},
-                                {v.location(),
-                                 "while inserting this array-of-tables"}
-                            }), v.location());
-                    }
-                    // the above if-else-if checks tab->at(k) is an array
-                    auto& a = tab->at(k).as_array();
-                    // If table element is defined as [[array_of_tables]], it
-                    // cannot be an empty array. If an array of tables is
-                    // defined as `aot = []`, it cannot be appended.
-                    if(a.empty() || !(a.front().is_table()))
-                    {
-                        throw syntax_error(format_underline(concat_to_string(
-                            "toml::insert_value: array of table (\"",
-                            format_dotted_keys(first, last), "\") collides with"
-                            " existing value"), {
-                                {tab->at(k).location(),
-                                 concat_to_string("this ", tab->at(k).type(),
-                                                  " value already exists")},
-                                {v.location(),
-                                 "while inserting this array-of-tables"}
-                            }), v.location());
-                    }
-                    // avoid conflicting array of table like the following.
-                    // ```toml
-                    // a = [{b = 42}] # define a as an array of *inline* tables
-                    // [[a]]          # a is an array of *multi-line* tables
-                    // b = 54
-                    // ```
-                    // Here, from the type information, these cannot be detected
-                    // because inline table is also a table.
-                    // But toml v0.5.0 explicitly says it is invalid. The above
-                    // array-of-tables has a static size and appending to the
-                    // array is invalid.
-                    // In this library, multi-line table value has a region
-                    // that points to the key of the table (e.g. [[a]]). By
-                    // comparing the first two letters in key, we can detect
-                    // the array-of-table is inline or multiline.
-                    if(const auto ptr = detail::get_region(a.front()))
-                    {
-                        if(ptr->str().substr(0,2) != "[[")
-                        {
-                            throw syntax_error(format_underline(concat_to_string(
-                                "toml::insert_value: array of table (\"",
-                                format_dotted_keys(first, last), "\") collides "
-                                "with existing array-of-tables"), {
-                                    {tab->at(k).location(),
-                                     concat_to_string("this ", tab->at(k).type(),
-                                                      " value has static size")},
-                                    {v.location(),
-                                     "appending it to the statically sized array"}
-                                }), v.location());
-                        }
-                    }
-                    a.push_back(v);
-                    return ok(true);
-                }
-                else // if not, we need to create the array of table
-                {
-                    // XXX: Consider the following array of tables.
-                    // ```toml
-                    // # This is a comment.
-                    // [[aot]]
-                    // foo = "bar"
-                    // ```
-                    // Here, the comment is for `aot`. But here, actually two
-                    // values are defined. An array that contains tables, named
-                    // `aot`, and the 0th element of the `aot`, `{foo = "bar"}`.
-                    // Those two are different from each other. But both of them
-                    // points to the same portion of the TOML file, `[[aot]]`,
-                    // so `key_reg.comments()` returns `# This is a comment`.
-                    // If it is assigned as a comment of `aot` defined here, the
-                    // comment will be duplicated. Both the `aot` itself and
-                    // the 0-th element will have the same comment. This causes
-                    // "duplication of the same comments" bug when the data is
-                    // serialized.
-                    //     Next, consider the following.
-                    // ```toml
-                    // # comment 1
-                    // aot = [
-                    //     # comment 2
-                    //     {foo = "bar"},
-                    // ]
-                    // ```
-                    // In this case, we can distinguish those two comments. So
-                    // here we need to add "comment 1" to the `aot` and
-                    // "comment 2" to the 0th element of that.
-                    //     To distinguish those two, we check the key region.
-                    std::vector<std::string> comments{/* empty by default */};
-                    if(key_reg.str().substr(0, 2) != "[[")
-                    {
-                        comments = key_reg.comments();
-                    }
-                    value_type aot(array_type(1, v), key_reg, std::move(comments));
-                    tab->insert(std::make_pair(k, aot));
-                    return ok(true);
-                }
-            } // end if(array of table)
-
-            if(tab->count(k) == 1)
-            {
-                if(tab->at(k).is_table() && v.is_table())
-                {
-                    if(!is_valid_forward_table_definition(
-                                tab->at(k), v, first, iter, last))
-                    {
-                        throw syntax_error(format_underline(concat_to_string(
-                            "toml::insert_value: table (\"",
-                            format_dotted_keys(first, last),
-                            "\") already exists."), {
-                                {tab->at(k).location(), "table already exists here"},
-                                {v.location(), "table defined twice"}
-                            }), v.location());
-                    }
-                    // to allow the following toml file.
-                    // [a.b.c]
-                    // d = 42
-                    // [a]
-                    // e = 2.71
-                    auto& t = tab->at(k).as_table();
-                    for(const auto& kv : v.as_table())
-                    {
-                        if(tab->at(k).contains(kv.first))
-                        {
-                            throw syntax_error(format_underline(concat_to_string(
-                                "toml::insert_value: value (\"",
-                                format_dotted_keys(first, last),
-                                "\") already exists."), {
-                                    {t.at(kv.first).location(), "already exists here"},
-                                    {v.location(), "this defined twice"}
-                                }), v.location());
-                        }
-                        t[kv.first] = kv.second;
-                    }
-                    detail::change_region(tab->at(k), key_reg);
-                    return ok(true);
-                }
-                else if(v.is_table()                     &&
-                        tab->at(k).is_array()            &&
-                        tab->at(k).as_array().size() > 0 &&
-                        tab->at(k).as_array().front().is_table())
-                {
-                    throw syntax_error(format_underline(concat_to_string(
-                        "toml::insert_value: array of tables (\"",
-                        format_dotted_keys(first, last), "\") already exists."), {
-                            {tab->at(k).location(), "array of tables defined here"},
-                            {v.location(), "table conflicts with the previous array of table"}
-                        }), v.location());
-                }
-                else
-                {
-                    throw syntax_error(format_underline(concat_to_string(
-                        "toml::insert_value: value (\"",
-                        format_dotted_keys(first, last), "\") already exists."), {
-                            {tab->at(k).location(), "value already exists here"},
-                            {v.location(), "value defined twice"}
-                        }), v.location());
-                }
-            }
-            tab->insert(std::make_pair(k, v));
-            return ok(true);
-        }
-        else // k is not the last one, we should insert recursively
-        {
-            // if there is no corresponding value, insert it first.
-            // related: you don't need to write
-            // # [x]
-            // # [x.y]
-            // to write
-            // [x.y.z]
-            if(tab->count(k) == 0)
-            {
-                // a table that is defined implicitly doesn't have any comments.
-                (*tab)[k] = value_type(table_type{}, key_reg, {/*no comment*/});
-            }
-
-            // type checking...
-            if(tab->at(k).is_table())
-            {
-                // According to toml-lang/toml:36d3091b3 "Clarify that inline
-                // tables are immutable", check if it adds key-value pair to an
-                // inline table.
-                if(const auto* ptr = get_region(tab->at(k)))
-                {
-                    // here, if the value is a (multi-line) table, the region
-                    // should be something like `[table-name]`.
-                    if(ptr->front() == '{')
-                    {
-                        throw syntax_error(format_underline(concat_to_string(
-                            "toml::insert_value: inserting to an inline table (",
-                            format_dotted_keys(first, std::next(iter)),
-                            ") but inline tables are immutable"), {
-                                {tab->at(k).location(), "inline tables are immutable"},
-                                {v.location(), "inserting this"}
-                            }), v.location());
-                    }
-                }
-                tab = std::addressof((*tab)[k].as_table());
-            }
-            else if(tab->at(k).is_array()) // inserting to array-of-tables?
-            {
-                auto& a = (*tab)[k].as_array();
-                if(!a.back().is_table())
-                {
-                    throw syntax_error(format_underline(concat_to_string(
-                        "toml::insert_value: target (",
-                        format_dotted_keys(first, std::next(iter)),
-                        ") is neither table nor an array of tables"), {
-                            {a.back().location(), concat_to_string(
-                                    "actual type is ", a.back().type())},
-                            {v.location(), "inserting this"}
-                        }), v.location());
-                }
-                tab = std::addressof(a.back().as_table());
-            }
-            else
-            {
-                throw syntax_error(format_underline(concat_to_string(
-                    "toml::insert_value: target (",
-                    format_dotted_keys(first, std::next(iter)),
-                    ") is neither table nor an array of tables"), {
-                        {tab->at(k).location(), concat_to_string(
-                                "actual type is ", tab->at(k).type())},
-                        {v.location(), "inserting this"}
-                    }), v.location());
-            }
-        }
-    }
-    return err(std::string("toml::detail::insert_nested_key: never reach here"));
-}
-
-template<typename Value>
-result<std::pair<typename Value::table_type, region>, std::string>
-parse_inline_table(location& loc)
-{
-    using value_type = Value;
-    using table_type = typename value_type::table_type;
-
-    const auto first = loc.iter();
-    table_type retval;
-    if(!(loc.iter() != loc.end() && *loc.iter() == '{'))
-    {
-        return err(format_underline("toml::parse_inline_table: ",
-            {{source_location(loc), "the next token is not an inline table"}}));
-    }
-    loc.advance();
-
-    // check if the inline table is an empty table = { }
-    maybe<lex_ws>::invoke(loc);
-    if(loc.iter() != loc.end() && *loc.iter() == '}')
-    {
-        loc.advance(); // skip `}`
-        return ok(std::make_pair(retval, region(loc, first, loc.iter())));
-    }
-
-    // it starts from "{". it should be formatted as inline-table
-    while(loc.iter() != loc.end())
-    {
-        const auto kv_r = parse_key_value_pair<value_type>(loc);
-        if(!kv_r)
-        {
-            return err(kv_r.unwrap_err());
-        }
-
-        const auto&              kvpair  = kv_r.unwrap();
-        const std::vector<key>&  keys    = kvpair.first.first;
-        const auto&              key_reg = kvpair.first.second;
-        const value_type&        val     = kvpair.second;
-
-        const auto inserted =
-            insert_nested_key(retval, val, keys.begin(), keys.end(), key_reg);
-        if(!inserted)
-        {
-            throw internal_error("toml::parse_inline_table: "
-                "failed to insert value into table: " + inserted.unwrap_err(),
-                source_location(loc));
-        }
-
-        using lex_table_separator = sequence<maybe<lex_ws>, character<','>>;
-        const auto sp = lex_table_separator::invoke(loc);
-
-        if(!sp)
-        {
-            maybe<lex_ws>::invoke(loc);
-
-            if(loc.iter() == loc.end())
-            {
-                throw syntax_error(format_underline(
-                    "toml::parse_inline_table: missing table separator `}` ",
-                    {{source_location(loc), "should be `}`"}}),
-                    source_location(loc));
-            }
-            else if(*loc.iter() == '}')
-            {
-                loc.advance(); // skip `}`
-                return ok(std::make_pair(
-                            retval, region(loc, first, loc.iter())));
-            }
-            else if(*loc.iter() == '#' || *loc.iter() == '\r' || *loc.iter() == '\n')
-            {
-                throw syntax_error(format_underline(
-                    "toml::parse_inline_table: missing curly brace `}`",
-                    {{source_location(loc), "should be `}`"}}),
-                    source_location(loc));
-            }
-            else
-            {
-                throw syntax_error(format_underline(
-                    "toml::parse_inline_table: missing table separator `,` ",
-                    {{source_location(loc), "should be `,`"}}),
-                    source_location(loc));
-            }
-        }
-        else // `,` is found
-        {
-            maybe<lex_ws>::invoke(loc);
-            if(loc.iter() != loc.end() && *loc.iter() == '}')
-            {
-                throw syntax_error(format_underline(
-                    "toml::parse_inline_table: trailing comma is not allowed in"
-                    " an inline table",
-                    {{source_location(loc), "should be `}`"}}),
-                    source_location(loc));
-            }
-        }
-    }
-    loc.reset(first);
-    throw syntax_error(format_underline("toml::parse_inline_table: "
-            "inline table did not closed by `}`",
-            {{source_location(loc), "should be closed"}}),
-            source_location(loc));
-}
-
-inline result<value_t, std::string> guess_number_type(const location& l)
-{
-    // This function tries to find some (common) mistakes by checking characters
-    // that follows the last character of a value. But it is often difficult
-    // because some non-newline characters can appear after a value. E.g.
-    // spaces, tabs, commas (in an array or inline table), closing brackets
-    // (of an array or inline table), comment-sign (#). Since this function
-    // does not parse further, those characters are always allowed to be there.
-    location loc = l;
-
-    if(lex_offset_date_time::invoke(loc)) {return ok(value_t::offset_datetime);}
-    loc.reset(l.iter());
-
-    if(lex_local_date_time::invoke(loc))
-    {
-        // bad offset may appear after this.
-        if(loc.iter() != loc.end() && (*loc.iter() == '+' || *loc.iter() == '-'
-                    || *loc.iter() == 'Z' || *loc.iter() == 'z'))
-        {
-            return err(format_underline("bad offset: should be [+-]HH:MM or Z",
-                        {{source_location(loc), "[+-]HH:MM or Z"}},
-                        {"pass: +09:00, -05:30", "fail: +9:00, -5:30"}));
-        }
-        return ok(value_t::local_datetime);
-    }
-    loc.reset(l.iter());
-
-    if(lex_local_date::invoke(loc))
-    {
-        // bad time may appear after this.
-        // A space is allowed as a delimiter between local time. But there are
-        // both cases in which a space becomes valid or invalid.
-        // - invalid: 2019-06-16 7:00:00
-        // - valid  : 2019-06-16 07:00:00
-        if(loc.iter() != loc.end())
-        {
-            const auto c = *loc.iter();
-            if(c == 'T' || c == 't')
-            {
-                return err(format_underline("bad time: should be HH:MM:SS.subsec",
-                        {{source_location(loc), "HH:MM:SS.subsec"}},
-                        {"pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999",
-                         "fail: 1979-05-27T7:32:00, 1979-05-27 17:32"}));
-            }
-            if('0' <= c && c <= '9')
-            {
-                return err(format_underline("bad time: missing T",
-                        {{source_location(loc), "T or space required here"}},
-                        {"pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999",
-                         "fail: 1979-05-27T7:32:00, 1979-05-27 7:32"}));
-            }
-            if(c == ' ' && std::next(loc.iter()) != loc.end() &&
-                ('0' <= *std::next(loc.iter()) && *std::next(loc.iter())<= '9'))
-            {
-                loc.advance();
-                return err(format_underline("bad time: should be HH:MM:SS.subsec",
-                        {{source_location(loc), "HH:MM:SS.subsec"}},
-                        {"pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999",
-                         "fail: 1979-05-27T7:32:00, 1979-05-27 7:32"}));
-            }
-        }
-        return ok(value_t::local_date);
-    }
-    loc.reset(l.iter());
-
-    if(lex_local_time::invoke(loc)) {return ok(value_t::local_time);}
-    loc.reset(l.iter());
-
-    if(lex_float::invoke(loc))
-    {
-        if(loc.iter() != loc.end() && *loc.iter() == '_')
-        {
-            return err(format_underline("bad float: `_` should be surrounded by digits",
-                        {{source_location(loc), "here"}},
-                        {"pass: +1.0, -2e-2, 3.141_592_653_589, inf, nan",
-                         "fail: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0"}));
-        }
-        return ok(value_t::floating);
-    }
-    loc.reset(l.iter());
-
-    if(lex_integer::invoke(loc))
-    {
-        if(loc.iter() != loc.end())
-        {
-            const auto c = *loc.iter();
-            if(c == '_')
-            {
-                return err(format_underline("bad integer: `_` should be surrounded by digits",
-                            {{source_location(loc), "here"}},
-                            {"pass: -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755",
-                             "fail: 1__000, 0123"}));
-            }
-            if('0' <= c && c <= '9')
-            {
-                // leading zero. point '0'
-                loc.retrace();
-                return err(format_underline("bad integer: leading zero",
-                            {{source_location(loc), "here"}},
-                            {"pass: -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755",
-                             "fail: 1__000, 0123"}));
-            }
-            if(c == ':' || c == '-')
-            {
-                return err(format_underline("bad datetime: invalid format",
-                            {{source_location(loc), "here"}},
-                            {"pass: 1979-05-27T07:32:00-07:00, 1979-05-27 07:32:00.999999Z",
-                             "fail: 1979-05-27T7:32:00-7:00, 1979-05-27 7:32-00:30"}));
-            }
-            if(c == '.' || c == 'e' || c == 'E')
-            {
-                return err(format_underline("bad float: invalid format",
-                            {{source_location(loc), "here"}},
-                            {"pass: +1.0, -2e-2, 3.141_592_653_589, inf, nan",
-                             "fail: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0"}));
-            }
-        }
-        return ok(value_t::integer);
-    }
-    if(loc.iter() != loc.end() && *loc.iter() == '.')
-    {
-        return err(format_underline("bad float: invalid format",
-                {{source_location(loc), "integer part required before this"}},
-                {"pass: +1.0, -2e-2, 3.141_592_653_589, inf, nan",
-                 "fail: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0"}));
-    }
-    if(loc.iter() != loc.end() && *loc.iter() == '_')
-    {
-        return err(format_underline("bad number: `_` should be surrounded by digits",
-                {{source_location(loc), "`_` is not surrounded by digits"}},
-                {"pass: -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755",
-                 "fail: 1__000, 0123"}));
-    }
-    return err(format_underline("bad format: unknown value appeared",
-                {{source_location(loc), "here"}}));
-}
-
-inline result<value_t, std::string> guess_value_type(const location& loc)
-{
-    switch(*loc.iter())
-    {
-        case '"' : {return ok(value_t::string);  }
-        case '\'': {return ok(value_t::string);  }
-        case 't' : {return ok(value_t::boolean); }
-        case 'f' : {return ok(value_t::boolean); }
-        case '[' : {return ok(value_t::array);   }
-        case '{' : {return ok(value_t::table);   }
-        case 'i' : {return ok(value_t::floating);} // inf.
-        case 'n' : {return ok(value_t::floating);} // nan.
-        default  : {return guess_number_type(loc);}
-    }
-}
-
-template<typename Value, typename T>
-result<Value, std::string>
-parse_value_helper(result<std::pair<T, region>, std::string> rslt)
-{
-    if(rslt.is_ok())
-    {
-        auto comments = rslt.as_ok().second.comments();
-        return ok(Value(std::move(rslt.as_ok()), std::move(comments)));
-    }
-    else
-    {
-        return err(std::move(rslt.as_err()));
-    }
-}
-
-template<typename Value>
-result<Value, std::string> parse_value(location& loc)
-{
-    const auto first = loc.iter();
-    if(first == loc.end())
-    {
-        return err(format_underline("toml::parse_value: input is empty",
-                   {{source_location(loc), ""}}));
-    }
-
-    const auto type = guess_value_type(loc);
-    if(!type)
-    {
-        return err(type.unwrap_err());
-    }
-
-    switch(type.unwrap())
-    {
-        case value_t::boolean        : {return parse_value_helper<Value>(parse_boolean(loc)            );}
-        case value_t::integer        : {return parse_value_helper<Value>(parse_integer(loc)            );}
-        case value_t::floating       : {return parse_value_helper<Value>(parse_floating(loc)           );}
-        case value_t::string         : {return parse_value_helper<Value>(parse_string(loc)             );}
-        case value_t::offset_datetime: {return parse_value_helper<Value>(parse_offset_datetime(loc)    );}
-        case value_t::local_datetime : {return parse_value_helper<Value>(parse_local_datetime(loc)     );}
-        case value_t::local_date     : {return parse_value_helper<Value>(parse_local_date(loc)         );}
-        case value_t::local_time     : {return parse_value_helper<Value>(parse_local_time(loc)         );}
-        case value_t::array          : {return parse_value_helper<Value>(parse_array<Value>(loc)       );}
-        case value_t::table          : {return parse_value_helper<Value>(parse_inline_table<Value>(loc));}
-        default:
-        {
-            const auto msg = format_underline("toml::parse_value: "
-                    "unknown token appeared", {{source_location(loc), "unknown"}});
-            loc.reset(first);
-            return err(msg);
-        }
-    }
-}
-
-inline result<std::pair<std::vector<key>, region>, std::string>
-parse_table_key(location& loc)
-{
-    if(auto token = lex_std_table::invoke(loc))
-    {
-        location inner_loc(loc.name(), token.unwrap().str());
-
-        const auto open = lex_std_table_open::invoke(inner_loc);
-        if(!open || inner_loc.iter() == inner_loc.end())
-        {
-            throw internal_error(format_underline(
-                "toml::parse_table_key: no `[`",
-                {{source_location(inner_loc), "should be `[`"}}),
-                source_location(inner_loc));
-        }
-        // to skip [ a . b . c ]
-        //          ^----------- this whitespace
-        lex_ws::invoke(inner_loc);
-        const auto keys = parse_key(inner_loc);
-        if(!keys)
-        {
-            throw internal_error(format_underline(
-                "toml::parse_table_key: invalid key",
-                {{source_location(inner_loc), "not key"}}),
-                source_location(inner_loc));
-        }
-        // to skip [ a . b . c ]
-        //                    ^-- this whitespace
-        lex_ws::invoke(inner_loc);
-        const auto close = lex_std_table_close::invoke(inner_loc);
-        if(!close)
-        {
-            throw internal_error(format_underline(
-                "toml::parse_table_key: no `]`",
-                {{source_location(inner_loc), "should be `]`"}}),
-                source_location(inner_loc));
-        }
-
-        // after [table.key], newline or EOF(empty table) required.
-        if(loc.iter() != loc.end())
-        {
-            using lex_newline_after_table_key =
-                sequence<maybe<lex_ws>, maybe<lex_comment>, lex_newline>;
-            const auto nl = lex_newline_after_table_key::invoke(loc);
-            if(!nl)
-            {
-                throw syntax_error(format_underline(
-                    "toml::parse_table_key: newline required after [table.key]",
-                    {{source_location(loc), "expected newline"}}),
-                    source_location(loc));
-            }
-        }
-        return ok(std::make_pair(keys.unwrap().first, token.unwrap()));
-    }
-    else
-    {
-        return err(format_underline("toml::parse_table_key: "
-            "not a valid table key", {{source_location(loc), "here"}}));
-    }
-}
-
-inline result<std::pair<std::vector<key>, region>, std::string>
-parse_array_table_key(location& loc)
-{
-    if(auto token = lex_array_table::invoke(loc))
-    {
-        location inner_loc(loc.name(), token.unwrap().str());
-
-        const auto open = lex_array_table_open::invoke(inner_loc);
-        if(!open || inner_loc.iter() == inner_loc.end())
-        {
-            throw internal_error(format_underline(
-                "toml::parse_array_table_key: no `[[`",
-                {{source_location(inner_loc), "should be `[[`"}}),
-                source_location(inner_loc));
-        }
-        lex_ws::invoke(inner_loc);
-        const auto keys = parse_key(inner_loc);
-        if(!keys)
-        {
-            throw internal_error(format_underline(
-                "toml::parse_array_table_key: invalid key",
-                {{source_location(inner_loc), "not a key"}}),
-                source_location(inner_loc));
-        }
-        lex_ws::invoke(inner_loc);
-        const auto close = lex_array_table_close::invoke(inner_loc);
-        if(!close)
-        {
-            throw internal_error(format_underline(
-                "toml::parse_table_key: no `]]`",
-                {{source_location(inner_loc), "should be `]]`"}}),
-                source_location(inner_loc));
-        }
-
-        // after [[table.key]], newline or EOF(empty table) required.
-        if(loc.iter() != loc.end())
-        {
-            using lex_newline_after_table_key =
-                sequence<maybe<lex_ws>, maybe<lex_comment>, lex_newline>;
-            const auto nl = lex_newline_after_table_key::invoke(loc);
-            if(!nl)
-            {
-                throw syntax_error(format_underline("toml::"
-                    "parse_array_table_key: newline required after [[table.key]]",
-                    {{source_location(loc), "expected newline"}}),
-                    source_location(loc));
-            }
-        }
-        return ok(std::make_pair(keys.unwrap().first, token.unwrap()));
-    }
-    else
-    {
-        return err(format_underline("toml::parse_array_table_key: "
-            "not a valid table key", {{source_location(loc), "here"}}));
-    }
-}
-
-// parse table body (key-value pairs until the iter hits the next [tablekey])
-template<typename Value>
-result<typename Value::table_type, std::string>
-parse_ml_table(location& loc)
-{
-    using value_type = Value;
-    using table_type = typename value_type::table_type;
-
-    const auto first = loc.iter();
-    if(first == loc.end())
-    {
-        return ok(table_type{});
-    }
-
-    // XXX at lest one newline is needed.
-    using skip_line = repeat<
-        sequence<maybe<lex_ws>, maybe<lex_comment>, lex_newline>, at_least<1>>;
-    skip_line::invoke(loc);
-    lex_ws::invoke(loc);
-
-    table_type tab;
-    while(loc.iter() != loc.end())
-    {
-        lex_ws::invoke(loc);
-        const auto before = loc.iter();
-        if(const auto tmp = parse_array_table_key(loc)) // next table found
-        {
-            loc.reset(before);
-            return ok(tab);
-        }
-        if(const auto tmp = parse_table_key(loc)) // next table found
-        {
-            loc.reset(before);
-            return ok(tab);
-        }
-
-        if(const auto kv = parse_key_value_pair<value_type>(loc))
-        {
-            const auto&              kvpair  = kv.unwrap();
-            const std::vector<key>&  keys    = kvpair.first.first;
-            const auto&              key_reg = kvpair.first.second;
-            const value_type&        val     = kvpair.second;
-            const auto inserted =
-                insert_nested_key(tab, val, keys.begin(), keys.end(), key_reg);
-            if(!inserted)
-            {
-                return err(inserted.unwrap_err());
-            }
-        }
-        else
-        {
-            return err(kv.unwrap_err());
-        }
-
-        // comment lines are skipped by the above function call.
-        // However, since the `skip_line` requires at least 1 newline, it fails
-        // if the file ends with ws and/or comment without newline.
-        // `skip_line` matches `ws? + comment? + newline`, not `ws` or `comment`
-        // itself. To skip the last ws and/or comment, call lexers.
-        // It does not matter if these fails, so the return value is discarded.
-        lex_ws::invoke(loc);
-        lex_comment::invoke(loc);
-
-        // skip_line is (whitespace? comment? newline)_{1,}. multiple empty lines
-        // and comments after the last key-value pairs are allowed.
-        const auto newline = skip_line::invoke(loc);
-        if(!newline && loc.iter() != loc.end())
-        {
-            const auto before2 = loc.iter();
-            lex_ws::invoke(loc); // skip whitespace
-            const auto msg = format_underline("toml::parse_table: "
-                "invalid line format", {{source_location(loc), concat_to_string(
-                "expected newline, but got '", show_char(*loc.iter()), "'.")}});
-            loc.reset(before2);
-            return err(msg);
-        }
-
-        // the skip_lines only matches with lines that includes newline.
-        // to skip the last line that includes comment and/or whitespace
-        // but no newline, call them one more time.
-        lex_ws::invoke(loc);
-        lex_comment::invoke(loc);
-    }
-    return ok(tab);
-}
-
-template<typename Value>
-result<Value, std::string> parse_toml_file(location& loc)
-{
-    using value_type = Value;
-    using table_type = typename value_type::table_type;
-
-    const auto first = loc.iter();
-    if(first == loc.end())
-    {
-        // For empty files, return an empty table with an empty region (zero-length).
-        // Without the region, error messages would miss the filename.
-        return ok(value_type(table_type{}, region(loc, first, first), {}));
-    }
-
-    // put the first line as a region of a file
-    // Here first != loc.end(), so taking std::next is okay
-    const region file(loc, first, std::next(loc.iter()));
-
-    // The first successive comments that are separated from the first value
-    // by an empty line are for a file itself.
-    // ```toml
-    // # this is a comment for a file.
-    //
-    // key = "the first value"
-    // ```
-    // ```toml
-    // # this is a comment for "the first value".
-    // key = "the first value"
-    // ```
-    std::vector<std::string> comments;
-    using lex_first_comments = sequence<
-        repeat<sequence<maybe<lex_ws>, lex_comment, lex_newline>, at_least<1>>,
-        sequence<maybe<lex_ws>, lex_newline>
-        >;
-    if(const auto token = lex_first_comments::invoke(loc))
-    {
-        location inner_loc(loc.name(), token.unwrap().str());
-        while(inner_loc.iter() != inner_loc.end())
-        {
-            maybe<lex_ws>::invoke(inner_loc); // remove ws if exists
-            if(lex_newline::invoke(inner_loc))
-            {
-                assert(inner_loc.iter() == inner_loc.end());
-                break; // empty line found.
-            }
-            auto com = lex_comment::invoke(inner_loc).unwrap().str();
-            com.erase(com.begin()); // remove # sign
-            comments.push_back(std::move(com));
-            lex_newline::invoke(inner_loc);
-        }
-    }
-
-    table_type data;
-    // root object is also a table, but without [tablename]
-    if(const auto tab = parse_ml_table<value_type>(loc))
-    {
-        data = std::move(tab.unwrap());
-    }
-    else // failed (empty table is regarded as success in parse_ml_table)
-    {
-        return err(tab.unwrap_err());
-    }
-    while(loc.iter() != loc.end())
-    {
-        // here, the region of [table] is regarded as the table-key because
-        // the table body is normally too big and it is not so informative
-        // if the first key-value pair of the table is shown in the error
-        // message.
-        if(const auto tabkey = parse_array_table_key(loc))
-        {
-            const auto tab = parse_ml_table<value_type>(loc);
-            if(!tab){return err(tab.unwrap_err());}
-
-            const auto& tk   = tabkey.unwrap();
-            const auto& keys = tk.first;
-            const auto& reg  = tk.second;
-
-            const auto inserted = insert_nested_key(data,
-                    value_type(tab.unwrap(), reg, reg.comments()),
-                    keys.begin(), keys.end(), reg,
-                    /*is_array_of_table=*/ true);
-            if(!inserted) {return err(inserted.unwrap_err());}
-
-            continue;
-        }
-        if(const auto tabkey = parse_table_key(loc))
-        {
-            const auto tab = parse_ml_table<value_type>(loc);
-            if(!tab){return err(tab.unwrap_err());}
-
-            const auto& tk   = tabkey.unwrap();
-            const auto& keys = tk.first;
-            const auto& reg  = tk.second;
-
-            const auto inserted = insert_nested_key(data,
-                value_type(tab.unwrap(), reg, reg.comments()),
-                keys.begin(), keys.end(), reg);
-            if(!inserted) {return err(inserted.unwrap_err());}
-
-            continue;
-        }
-        return err(format_underline("toml::parse_toml_file: "
-            "unknown line appeared", {{source_location(loc), "unknown format"}}));
-    }
-
-    return ok(Value(std::move(data), file, comments));
-}
-
-} // detail
-
-template<typename                     Comment = TOML11_DEFAULT_COMMENT_STRATEGY,
-         template<typename ...> class Table   = std::unordered_map,
-         template<typename ...> class Array   = std::vector>
-basic_value<Comment, Table, Array>
-parse(std::istream& is, const std::string& fname = "unknown file")
-{
-    using value_type = basic_value<Comment, Table, Array>;
-
-    const auto beg = is.tellg();
-    is.seekg(0, std::ios::end);
-    const auto end = is.tellg();
-    const auto fsize = end - beg;
-    is.seekg(beg);
-
-    // read whole file as a sequence of char
-    assert(fsize >= 0);
-    std::vector<char> letters(static_cast<std::size_t>(fsize));
-    is.read(letters.data(), fsize);
-
-    // append LF.
-    // Although TOML does not require LF at the EOF, to make parsing logic
-    // simpler, we "normalize" the content by adding LF if it does not exist.
-    // It also checks if the last char is CR, to avoid changing the meaning.
-    // This is not the *best* way to deal with the last character, but is a
-    // simple and quick fix.
-    if(!letters.empty() && letters.back() != '\n' && letters.back() != '\r')
-    {
-        letters.push_back('\n');
-    }
-
-    detail::location loc(std::move(fname), std::move(letters));
-
-    // skip BOM if exists.
-    // XXX component of BOM (like 0xEF) exceeds the representable range of
-    // signed char, so on some (actually, most) of the environment, these cannot
-    // be compared to char. However, since we are always out of luck, we need to
-    // check our chars are equivalent to BOM. To do this, first we need to
-    // convert char to unsigned char to guarantee the comparability.
-    if(loc.source()->size() >= 3)
-    {
-        std::array<unsigned char, 3> BOM;
-        std::memcpy(BOM.data(), loc.source()->data(), 3);
-        if(BOM[0] == 0xEF && BOM[1] == 0xBB && BOM[2] == 0xBF)
-        {
-            loc.advance(3); // BOM found. skip.
-        }
-    }
-
-    const auto data = detail::parse_toml_file<value_type>(loc);
-    if(!data)
-    {
-        throw syntax_error(data.unwrap_err(), source_location(loc));
-    }
-    return data.unwrap();
-}
-
-template<typename                     Comment = TOML11_DEFAULT_COMMENT_STRATEGY,
-         template<typename ...> class Table   = std::unordered_map,
-         template<typename ...> class Array   = std::vector>
-basic_value<Comment, Table, Array> parse(const std::string& fname)
-{
-    std::ifstream ifs(fname.c_str(), std::ios_base::binary);
-    if(!ifs.good())
-    {
-        throw std::runtime_error("toml::parse: file open error -> " + fname);
-    }
-    return parse<Comment, Table, Array>(ifs, fname);
-}
-
-#ifdef TOML11_HAS_STD_FILESYSTEM
-// This function just forwards `parse("filename.toml")` to std::string version
-// to avoid the ambiguity in overload resolution.
-//
-// Both std::string and std::filesystem::path are convertible from const char*.
-// Without this, both parse(std::string) and parse(std::filesystem::path)
-// matches to parse("filename.toml"). This breaks the existing code.
-//
-// This function exactly matches to the invocation with c-string.
-// So this function is preferred than others and the ambiguity disappears.
-template<typename                     Comment = TOML11_DEFAULT_COMMENT_STRATEGY,
-         template<typename ...> class Table   = std::unordered_map,
-         template<typename ...> class Array   = std::vector>
-basic_value<Comment, Table, Array> parse(const char* fname)
-{
-    return parse<Comment, Table, Array>(std::string(fname));
-}
-
-template<typename                     Comment = TOML11_DEFAULT_COMMENT_STRATEGY,
-         template<typename ...> class Table   = std::unordered_map,
-         template<typename ...> class Array   = std::vector>
-basic_value<Comment, Table, Array> parse(const std::filesystem::path& fpath)
-{
-    std::ifstream ifs(fpath, std::ios_base::binary);
-    if(!ifs.good())
-    {
-        throw std::runtime_error("toml::parse: file open error -> " +
-                                 fpath.string());
-    }
-    return parse<Comment, Table, Array>(ifs, fpath.string());
-}
-#endif // TOML11_HAS_STD_FILESYSTEM
-
-} // toml
-#endif// TOML11_PARSER_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/region.hpp b/src/frontend/qt_sdl/toml/toml/region.hpp
deleted file mode 100644
index 2e01e51d..00000000
--- a/src/frontend/qt_sdl/toml/toml/region.hpp
+++ /dev/null
@@ -1,417 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_REGION_HPP
-#define TOML11_REGION_HPP
-#include <memory>
-#include <vector>
-#include <algorithm>
-#include <initializer_list>
-#include <iterator>
-#include <iomanip>
-#include <cassert>
-#include "color.hpp"
-
-namespace toml
-{
-namespace detail
-{
-
-// helper function to avoid std::string(0, 'c') or std::string(iter, iter)
-template<typename Iterator>
-std::string make_string(Iterator first, Iterator last)
-{
-    if(first == last) {return "";}
-    return std::string(first, last);
-}
-inline std::string make_string(std::size_t len, char c)
-{
-    if(len == 0) {return "";}
-    return std::string(len, c);
-}
-
-// region_base is a base class of location and region that are defined below.
-// it will be used to generate better error messages.
-struct region_base
-{
-    region_base() = default;
-    virtual ~region_base() = default;
-    region_base(const region_base&) = default;
-    region_base(region_base&&     ) = default;
-    region_base& operator=(const region_base&) = default;
-    region_base& operator=(region_base&&     ) = default;
-
-    virtual bool is_ok()           const noexcept {return false;}
-    virtual char front()           const noexcept {return '\0';}
-
-    virtual std::string str()      const {return std::string("unknown region");}
-    virtual std::string name()     const {return std::string("unknown file");}
-    virtual std::string line()     const {return std::string("unknown line");}
-    virtual std::string line_num() const {return std::string("?");}
-
-    // length of the region
-    virtual std::size_t size()     const noexcept {return 0;}
-    // number of characters in the line before the region
-    virtual std::size_t before()   const noexcept {return 0;}
-    // number of characters in the line after the region
-    virtual std::size_t after()    const noexcept {return 0;}
-
-    virtual std::vector<std::string> comments() const {return {};}
-    // ```toml
-    // # comment_before
-    // key = "value" # comment_inline
-    // ```
-};
-
-// location represents a position in a container, which contains a file content.
-// it can be considered as a region that contains only one character.
-//
-// it contains pointer to the file content and iterator that points the current
-// location.
-struct location final : public region_base
-{
-    using const_iterator  = typename std::vector<char>::const_iterator;
-    using difference_type = typename const_iterator::difference_type;
-    using source_ptr      = std::shared_ptr<const std::vector<char>>;
-
-    location(std::string source_name, std::vector<char> cont)
-      : source_(std::make_shared<std::vector<char>>(std::move(cont))),
-        line_number_(1), source_name_(std::move(source_name)), iter_(source_->cbegin())
-    {}
-    location(std::string source_name, const std::string& cont)
-      : source_(std::make_shared<std::vector<char>>(cont.begin(), cont.end())),
-        line_number_(1), source_name_(std::move(source_name)), iter_(source_->cbegin())
-    {}
-
-    location(const location&) = default;
-    location(location&&)      = default;
-    location& operator=(const location&) = default;
-    location& operator=(location&&)      = default;
-    ~location() = default;
-
-    bool is_ok() const noexcept override {return static_cast<bool>(source_);}
-    char front() const noexcept override {return *iter_;}
-
-    // this const prohibits codes like `++(loc.iter())`.
-    const const_iterator iter()  const noexcept {return iter_;}
-
-    const_iterator begin() const noexcept {return source_->cbegin();}
-    const_iterator end()   const noexcept {return source_->cend();}
-
-    // XXX `location::line_num()` used to be implemented using `std::count` to
-    // count a number of '\n'. But with a long toml file (typically, 10k lines),
-    // it becomes intolerably slow because each time it generates error messages,
-    // it counts '\n' from thousands of characters. To workaround it, I decided
-    // to introduce `location::line_number_` member variable and synchronize it
-    // to the location changes the point to look. So an overload of `iter()`
-    // which returns mutable reference is removed and `advance()`, `retrace()`
-    // and `reset()` is added.
-    void advance(difference_type n = 1) noexcept
-    {
-        this->line_number_ += static_cast<std::size_t>(
-                std::count(this->iter_, std::next(this->iter_, n), '\n'));
-        this->iter_ += n;
-        return;
-    }
-    void retrace(difference_type n = 1) noexcept
-    {
-        this->line_number_ -= static_cast<std::size_t>(
-                std::count(std::prev(this->iter_, n), this->iter_, '\n'));
-        this->iter_ -= n;
-        return;
-    }
-    void reset(const_iterator rollback) noexcept
-    {
-        // since c++11, std::distance works in both ways for random-access
-        // iterators and returns a negative value if `first > last`.
-        if(0 <= std::distance(rollback, this->iter_)) // rollback < iter
-        {
-            this->line_number_ -= static_cast<std::size_t>(
-                    std::count(rollback, this->iter_, '\n'));
-        }
-        else // iter < rollback [[unlikely]]
-        {
-            this->line_number_ += static_cast<std::size_t>(
-                    std::count(this->iter_, rollback, '\n'));
-        }
-        this->iter_ = rollback;
-        return;
-    }
-
-    std::string str()  const override {return make_string(1, *this->iter());}
-    std::string name() const override {return source_name_;}
-
-    std::string line_num() const override
-    {
-        return std::to_string(this->line_number_);
-    }
-
-    std::string line() const override
-    {
-        return make_string(this->line_begin(), this->line_end());
-    }
-
-    const_iterator line_begin() const noexcept
-    {
-        using reverse_iterator = std::reverse_iterator<const_iterator>;
-        return std::find(reverse_iterator(this->iter()),
-                         reverse_iterator(this->begin()), '\n').base();
-    }
-    const_iterator line_end() const noexcept
-    {
-        return std::find(this->iter(), this->end(), '\n');
-    }
-
-    // location is always points a character. so the size is 1.
-    std::size_t size() const noexcept override
-    {
-        return 1u;
-    }
-    std::size_t before() const noexcept override
-    {
-        const auto sz = std::distance(this->line_begin(), this->iter());
-        assert(sz >= 0);
-        return static_cast<std::size_t>(sz);
-    }
-    std::size_t after() const noexcept override
-    {
-        const auto sz = std::distance(this->iter(), this->line_end());
-        assert(sz >= 0);
-        return static_cast<std::size_t>(sz);
-    }
-
-    source_ptr const& source() const& noexcept {return source_;}
-    source_ptr&&      source() &&     noexcept {return std::move(source_);}
-
-  private:
-
-    source_ptr     source_;
-    std::size_t    line_number_;
-    std::string    source_name_;
-    const_iterator iter_;
-};
-
-// region represents a range in a container, which contains a file content.
-//
-// it contains pointer to the file content and iterator that points the first
-// and last location.
-struct region final : public region_base
-{
-    using const_iterator = typename std::vector<char>::const_iterator;
-    using source_ptr     = std::shared_ptr<const std::vector<char>>;
-
-    // delete default constructor. source_ never be null.
-    region() = delete;
-
-    explicit region(const location& loc)
-      : source_(loc.source()), source_name_(loc.name()),
-        first_(loc.iter()), last_(loc.iter())
-    {}
-    explicit region(location&& loc)
-      : source_(loc.source()), source_name_(loc.name()),
-        first_(loc.iter()), last_(loc.iter())
-    {}
-
-    region(const location& loc, const_iterator f, const_iterator l)
-      : source_(loc.source()), source_name_(loc.name()), first_(f), last_(l)
-    {}
-    region(location&& loc, const_iterator f, const_iterator l)
-      : source_(loc.source()), source_name_(loc.name()), first_(f), last_(l)
-    {}
-
-    region(const region&) = default;
-    region(region&&)      = default;
-    region& operator=(const region&) = default;
-    region& operator=(region&&)      = default;
-    ~region() = default;
-
-    region& operator+=(const region& other)
-    {
-        // different regions cannot be concatenated
-        assert(this->begin() == other.begin() && this->end() == other.end() &&
-               this->last_   == other.first_);
-
-        this->last_ = other.last_;
-        return *this;
-    }
-
-    bool is_ok() const noexcept override {return static_cast<bool>(source_);}
-    char front() const noexcept override {return *first_;}
-
-    std::string str()  const override {return make_string(first_, last_);}
-    std::string line() const override
-    {
-        if(this->contain_newline())
-        {
-            return make_string(this->line_begin(),
-                    std::find(this->line_begin(), this->last(), '\n'));
-        }
-        return make_string(this->line_begin(), this->line_end());
-    }
-    std::string line_num() const override
-    {
-        return std::to_string(1 + std::count(this->begin(), this->first(), '\n'));
-    }
-
-    std::size_t size() const noexcept override
-    {
-        const auto sz = std::distance(first_, last_);
-        assert(sz >= 0);
-        return static_cast<std::size_t>(sz);
-    }
-    std::size_t before() const noexcept override
-    {
-        const auto sz = std::distance(this->line_begin(), this->first());
-        assert(sz >= 0);
-        return static_cast<std::size_t>(sz);
-    }
-    std::size_t after() const noexcept override
-    {
-        const auto sz = std::distance(this->last(), this->line_end());
-        assert(sz >= 0);
-        return static_cast<std::size_t>(sz);
-    }
-
-    bool contain_newline() const noexcept
-    {
-        return std::find(this->first(), this->last(), '\n') != this->last();
-    }
-
-    const_iterator line_begin() const noexcept
-    {
-        using reverse_iterator = std::reverse_iterator<const_iterator>;
-        return std::find(reverse_iterator(this->first()),
-                         reverse_iterator(this->begin()), '\n').base();
-    }
-    const_iterator line_end() const noexcept
-    {
-        return std::find(this->last(), this->end(), '\n');
-    }
-
-    const_iterator begin() const noexcept {return source_->cbegin();}
-    const_iterator end()   const noexcept {return source_->cend();}
-    const_iterator first() const noexcept {return first_;}
-    const_iterator last()  const noexcept {return last_;}
-
-    source_ptr const& source() const& noexcept {return source_;}
-    source_ptr&&      source() &&     noexcept {return std::move(source_);}
-
-    std::string name() const override {return source_name_;}
-
-    std::vector<std::string> comments() const override
-    {
-        // assuming the current region (`*this`) points a value.
-        // ```toml
-        // a = "value"
-        //     ^^^^^^^- this region
-        // ```
-        using rev_iter = std::reverse_iterator<const_iterator>;
-
-        std::vector<std::string> com{};
-        {
-            // find comments just before the current region.
-            // ```toml
-            // # this should be collected.
-            // # this also.
-            // a = value # not this.
-            // ```
-
-            // # this is a comment for `a`, not array elements.
-            // a = [1, 2, 3, 4, 5]
-            if(this->first() == std::find_if(this->line_begin(), this->first(),
-                [](const char c) noexcept -> bool {return c == '[' || c == '{';}))
-            {
-                auto iter = this->line_begin(); // points the first character
-                while(iter != this->begin())
-                {
-                    iter = std::prev(iter);
-
-                    // range [line_start, iter) represents the previous line
-                    const auto line_start   = std::find(
-                            rev_iter(iter), rev_iter(this->begin()), '\n').base();
-                    const auto comment_found = std::find(line_start, iter, '#');
-                    if(comment_found == iter)
-                    {
-                        break; // comment not found.
-                    }
-
-                    // exclude the following case.
-                    // > a = "foo" # comment // <-- this is not a comment for b but a.
-                    // > b = "current value"
-                    if(std::all_of(line_start, comment_found,
-                            [](const char c) noexcept -> bool {
-                                return c == ' ' || c == '\t';
-                            }))
-                    {
-                        // unwrap the first '#' by std::next.
-                        auto s = make_string(std::next(comment_found), iter);
-                        if(!s.empty() && s.back() == '\r') {s.pop_back();}
-                        com.push_back(std::move(s));
-                    }
-                    else
-                    {
-                        break;
-                    }
-                    iter = line_start;
-                }
-            }
-        }
-
-        if(com.size() > 1)
-        {
-            std::reverse(com.begin(), com.end());
-        }
-
-        {
-            // find comments just after the current region.
-            // ```toml
-            // # not this.
-            // a = value # this one.
-            // a = [ # not this (technically difficult)
-            //
-            // ] # and this.
-            // ```
-            // The reason why it's difficult is that it requires parsing in the
-            // following case.
-            // ```toml
-            // a = [ 10 # this comment is for `10`. not for `a` but `a[0]`.
-            // # ...
-            // ] # this is apparently a comment for a.
-            //
-            // b = [
-            // 3.14 ] # there is no way to add a comment to `3.14` currently.
-            //
-            // c = [
-            //   3.14 # do this if you need a comment here.
-            // ]
-            // ```
-            const auto comment_found =
-                std::find(this->last(), this->line_end(), '#');
-            if(comment_found != this->line_end()) // '#' found
-            {
-                // table = {key = "value"} # what is this for?
-                // the above comment is not for "value", but {key="value"}.
-                if(comment_found == std::find_if(this->last(), comment_found,
-                    [](const char c) noexcept -> bool {
-                        return !(c == ' ' || c == '\t' || c == ',');
-                    }))
-                {
-                    // unwrap the first '#' by std::next.
-                    auto s = make_string(std::next(comment_found), this->line_end());
-                    if(!s.empty() && s.back() == '\r') {s.pop_back();}
-                    com.push_back(std::move(s));
-                }
-            }
-        }
-        return com;
-    }
-
-  private:
-
-    source_ptr     source_;
-    std::string    source_name_;
-    const_iterator first_, last_;
-};
-
-} // detail
-} // toml
-#endif// TOML11_REGION_H
diff --git a/src/frontend/qt_sdl/toml/toml/result.hpp b/src/frontend/qt_sdl/toml/toml/result.hpp
deleted file mode 100644
index 77cd46c6..00000000
--- a/src/frontend/qt_sdl/toml/toml/result.hpp
+++ /dev/null
@@ -1,717 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_RESULT_HPP
-#define TOML11_RESULT_HPP
-#include "traits.hpp"
-#include <type_traits>
-#include <stdexcept>
-#include <utility>
-#include <new>
-#include <string>
-#include <sstream>
-#include <cassert>
-
-namespace toml
-{
-
-template<typename T>
-struct success
-{
-    using value_type = T;
-    value_type value;
-
-    explicit success(const value_type& v)
-        noexcept(std::is_nothrow_copy_constructible<value_type>::value)
-        : value(v)
-    {}
-    explicit success(value_type&& v)
-        noexcept(std::is_nothrow_move_constructible<value_type>::value)
-        : value(std::move(v))
-    {}
-
-    template<typename U>
-    explicit success(U&& v): value(std::forward<U>(v)) {}
-
-    template<typename U>
-    explicit success(const success<U>& v): value(v.value) {}
-    template<typename U>
-    explicit success(success<U>&& v): value(std::move(v.value)) {}
-
-    ~success() = default;
-    success(const success&) = default;
-    success(success&&)      = default;
-    success& operator=(const success&) = default;
-    success& operator=(success&&)      = default;
-};
-
-template<typename T>
-struct failure
-{
-    using value_type = T;
-    value_type value;
-
-    explicit failure(const value_type& v)
-        noexcept(std::is_nothrow_copy_constructible<value_type>::value)
-        : value(v)
-    {}
-    explicit failure(value_type&& v)
-        noexcept(std::is_nothrow_move_constructible<value_type>::value)
-        : value(std::move(v))
-    {}
-
-    template<typename U>
-    explicit failure(U&& v): value(std::forward<U>(v)) {}
-
-    template<typename U>
-    explicit failure(const failure<U>& v): value(v.value) {}
-    template<typename U>
-    explicit failure(failure<U>&& v): value(std::move(v.value)) {}
-
-    ~failure() = default;
-    failure(const failure&) = default;
-    failure(failure&&)      = default;
-    failure& operator=(const failure&) = default;
-    failure& operator=(failure&&)      = default;
-};
-
-template<typename T>
-success<typename std::remove_cv<typename std::remove_reference<T>::type>::type>
-ok(T&& v)
-{
-    return success<
-        typename std::remove_cv<typename std::remove_reference<T>::type>::type
-        >(std::forward<T>(v));
-}
-template<typename T>
-failure<typename std::remove_cv<typename std::remove_reference<T>::type>::type>
-err(T&& v)
-{
-    return failure<
-        typename std::remove_cv<typename std::remove_reference<T>::type>::type
-        >(std::forward<T>(v));
-}
-
-inline success<std::string> ok(const char* literal)
-{
-    return success<std::string>(std::string(literal));
-}
-inline failure<std::string> err(const char* literal)
-{
-    return failure<std::string>(std::string(literal));
-}
-
-
-template<typename T, typename E>
-struct result
-{
-    using value_type = T;
-    using error_type = E;
-    using success_type = success<value_type>;
-    using failure_type = failure<error_type>;
-
-    result(const success_type& s): is_ok_(true)
-    {
-        auto tmp = ::new(std::addressof(this->succ)) success_type(s);
-        assert(tmp == std::addressof(this->succ));
-        (void)tmp;
-    }
-    result(const failure_type& f): is_ok_(false)
-    {
-        auto tmp = ::new(std::addressof(this->fail)) failure_type(f);
-        assert(tmp == std::addressof(this->fail));
-        (void)tmp;
-    }
-    result(success_type&& s): is_ok_(true)
-    {
-        auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(s));
-        assert(tmp == std::addressof(this->succ));
-        (void)tmp;
-    }
-    result(failure_type&& f): is_ok_(false)
-    {
-        auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(f));
-        assert(tmp == std::addressof(this->fail));
-        (void)tmp;
-    }
-
-    template<typename U>
-    result(const success<U>& s): is_ok_(true)
-    {
-        auto tmp = ::new(std::addressof(this->succ)) success_type(s.value);
-        assert(tmp == std::addressof(this->succ));
-        (void)tmp;
-    }
-    template<typename U>
-    result(const failure<U>& f): is_ok_(false)
-    {
-        auto tmp = ::new(std::addressof(this->fail)) failure_type(f.value);
-        assert(tmp == std::addressof(this->fail));
-        (void)tmp;
-    }
-    template<typename U>
-    result(success<U>&& s): is_ok_(true)
-    {
-        auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(s.value));
-        assert(tmp == std::addressof(this->succ));
-        (void)tmp;
-    }
-    template<typename U>
-    result(failure<U>&& f): is_ok_(false)
-    {
-        auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(f.value));
-        assert(tmp == std::addressof(this->fail));
-        (void)tmp;
-    }
-
-    result& operator=(const success_type& s)
-    {
-        this->cleanup();
-        this->is_ok_ = true;
-        auto tmp = ::new(std::addressof(this->succ)) success_type(s);
-        assert(tmp == std::addressof(this->succ));
-        (void)tmp;
-        return *this;
-    }
-    result& operator=(const failure_type& f)
-    {
-        this->cleanup();
-        this->is_ok_ = false;
-        auto tmp = ::new(std::addressof(this->fail)) failure_type(f);
-        assert(tmp == std::addressof(this->fail));
-        (void)tmp;
-        return *this;
-    }
-    result& operator=(success_type&& s)
-    {
-        this->cleanup();
-        this->is_ok_ = true;
-        auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(s));
-        assert(tmp == std::addressof(this->succ));
-        (void)tmp;
-        return *this;
-    }
-    result& operator=(failure_type&& f)
-    {
-        this->cleanup();
-        this->is_ok_ = false;
-        auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(f));
-        assert(tmp == std::addressof(this->fail));
-        (void)tmp;
-        return *this;
-    }
-
-    template<typename U>
-    result& operator=(const success<U>& s)
-    {
-        this->cleanup();
-        this->is_ok_ = true;
-        auto tmp = ::new(std::addressof(this->succ)) success_type(s.value);
-        assert(tmp == std::addressof(this->succ));
-        (void)tmp;
-        return *this;
-    }
-    template<typename U>
-    result& operator=(const failure<U>& f)
-    {
-        this->cleanup();
-        this->is_ok_ = false;
-        auto tmp = ::new(std::addressof(this->fail)) failure_type(f.value);
-        assert(tmp == std::addressof(this->fail));
-        (void)tmp;
-        return *this;
-    }
-    template<typename U>
-    result& operator=(success<U>&& s)
-    {
-        this->cleanup();
-        this->is_ok_ = true;
-        auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(s.value));
-        assert(tmp == std::addressof(this->succ));
-        (void)tmp;
-        return *this;
-    }
-    template<typename U>
-    result& operator=(failure<U>&& f)
-    {
-        this->cleanup();
-        this->is_ok_ = false;
-        auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(f.value));
-        assert(tmp == std::addressof(this->fail));
-        (void)tmp;
-        return *this;
-    }
-
-    ~result() noexcept {this->cleanup();}
-
-    result(const result& other): is_ok_(other.is_ok())
-    {
-        if(other.is_ok())
-        {
-            auto tmp = ::new(std::addressof(this->succ)) success_type(other.as_ok());
-            assert(tmp == std::addressof(this->succ));
-            (void)tmp;
-        }
-        else
-        {
-            auto tmp = ::new(std::addressof(this->fail)) failure_type(other.as_err());
-            assert(tmp == std::addressof(this->fail));
-            (void)tmp;
-        }
-    }
-    result(result&& other): is_ok_(other.is_ok())
-    {
-        if(other.is_ok())
-        {
-            auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(other.as_ok()));
-            assert(tmp == std::addressof(this->succ));
-            (void)tmp;
-        }
-        else
-        {
-            auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(other.as_err()));
-            assert(tmp == std::addressof(this->fail));
-            (void)tmp;
-        }
-    }
-
-    template<typename U, typename F>
-    result(const result<U, F>& other): is_ok_(other.is_ok())
-    {
-        if(other.is_ok())
-        {
-            auto tmp = ::new(std::addressof(this->succ)) success_type(other.as_ok());
-            assert(tmp == std::addressof(this->succ));
-            (void)tmp;
-        }
-        else
-        {
-            auto tmp = ::new(std::addressof(this->fail)) failure_type(other.as_err());
-            assert(tmp == std::addressof(this->fail));
-            (void)tmp;
-        }
-    }
-    template<typename U, typename F>
-    result(result<U, F>&& other): is_ok_(other.is_ok())
-    {
-        if(other.is_ok())
-        {
-            auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(other.as_ok()));
-            assert(tmp == std::addressof(this->succ));
-            (void)tmp;
-        }
-        else
-        {
-            auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(other.as_err()));
-            assert(tmp == std::addressof(this->fail));
-            (void)tmp;
-        }
-    }
-
-    result& operator=(const result& other)
-    {
-        this->cleanup();
-        if(other.is_ok())
-        {
-            auto tmp = ::new(std::addressof(this->succ)) success_type(other.as_ok());
-            assert(tmp == std::addressof(this->succ));
-            (void)tmp;
-        }
-        else
-        {
-            auto tmp = ::new(std::addressof(this->fail)) failure_type(other.as_err());
-            assert(tmp == std::addressof(this->fail));
-            (void)tmp;
-        }
-        is_ok_ = other.is_ok();
-        return *this;
-    }
-    result& operator=(result&& other)
-    {
-        this->cleanup();
-        if(other.is_ok())
-        {
-            auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(other.as_ok()));
-            assert(tmp == std::addressof(this->succ));
-            (void)tmp;
-        }
-        else
-        {
-            auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(other.as_err()));
-            assert(tmp == std::addressof(this->fail));
-            (void)tmp;
-        }
-        is_ok_ = other.is_ok();
-        return *this;
-    }
-
-    template<typename U, typename F>
-    result& operator=(const result<U, F>& other)
-    {
-        this->cleanup();
-        if(other.is_ok())
-        {
-            auto tmp = ::new(std::addressof(this->succ)) success_type(other.as_ok());
-            assert(tmp == std::addressof(this->succ));
-            (void)tmp;
-        }
-        else
-        {
-            auto tmp = ::new(std::addressof(this->fail)) failure_type(other.as_err());
-            assert(tmp == std::addressof(this->fail));
-            (void)tmp;
-        }
-        is_ok_ = other.is_ok();
-        return *this;
-    }
-    template<typename U, typename F>
-    result& operator=(result<U, F>&& other)
-    {
-        this->cleanup();
-        if(other.is_ok())
-        {
-            auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(other.as_ok()));
-            assert(tmp == std::addressof(this->succ));
-            (void)tmp;
-        }
-        else
-        {
-            auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(other.as_err()));
-            assert(tmp == std::addressof(this->fail));
-            (void)tmp;
-        }
-        is_ok_ = other.is_ok();
-        return *this;
-    }
-
-    bool is_ok()  const noexcept {return is_ok_;}
-    bool is_err() const noexcept {return !is_ok_;}
-
-    operator bool() const noexcept {return is_ok_;}
-
-    value_type&       unwrap() &
-    {
-        if(is_err())
-        {
-            throw std::runtime_error("toml::result: bad unwrap: " +
-                                     format_error(this->as_err()));
-        }
-        return this->succ.value;
-    }
-    value_type const& unwrap() const&
-    {
-        if(is_err())
-        {
-            throw std::runtime_error("toml::result: bad unwrap: " +
-                                     format_error(this->as_err()));
-        }
-        return this->succ.value;
-    }
-    value_type&&      unwrap() &&
-    {
-        if(is_err())
-        {
-            throw std::runtime_error("toml::result: bad unwrap: " +
-                                     format_error(this->as_err()));
-        }
-        return std::move(this->succ.value);
-    }
-
-    value_type&       unwrap_or(value_type& opt) &
-    {
-        if(is_err()) {return opt;}
-        return this->succ.value;
-    }
-    value_type const& unwrap_or(value_type const& opt) const&
-    {
-        if(is_err()) {return opt;}
-        return this->succ.value;
-    }
-    value_type        unwrap_or(value_type opt) &&
-    {
-        if(is_err()) {return opt;}
-        return this->succ.value;
-    }
-
-    error_type&       unwrap_err() &
-    {
-        if(is_ok()) {throw std::runtime_error("toml::result: bad unwrap_err");}
-        return this->fail.value;
-    }
-    error_type const& unwrap_err() const&
-    {
-        if(is_ok()) {throw std::runtime_error("toml::result: bad unwrap_err");}
-        return this->fail.value;
-    }
-    error_type&&      unwrap_err() &&
-    {
-        if(is_ok()) {throw std::runtime_error("toml::result: bad unwrap_err");}
-        return std::move(this->fail.value);
-    }
-
-    value_type&       as_ok() &      noexcept {return this->succ.value;}
-    value_type const& as_ok() const& noexcept {return this->succ.value;}
-    value_type&&      as_ok() &&     noexcept {return std::move(this->succ.value);}
-
-    error_type&       as_err() &      noexcept {return this->fail.value;}
-    error_type const& as_err() const& noexcept {return this->fail.value;}
-    error_type&&      as_err() &&     noexcept {return std::move(this->fail.value);}
-
-
-    // prerequisities
-    // F: T -> U
-    // retval: result<U, E>
-    template<typename F>
-    result<detail::return_type_of_t<F, value_type&>, error_type>
-    map(F&& f) &
-    {
-        if(this->is_ok()){return ok(f(this->as_ok()));}
-        return err(this->as_err());
-    }
-    template<typename F>
-    result<detail::return_type_of_t<F, value_type const&>, error_type>
-    map(F&& f) const&
-    {
-        if(this->is_ok()){return ok(f(this->as_ok()));}
-        return err(this->as_err());
-    }
-    template<typename F>
-    result<detail::return_type_of_t<F, value_type &&>, error_type>
-    map(F&& f) &&
-    {
-        if(this->is_ok()){return ok(f(std::move(this->as_ok())));}
-        return err(std::move(this->as_err()));
-    }
-
-    // prerequisities
-    // F: E -> F
-    // retval: result<T, F>
-    template<typename F>
-    result<value_type, detail::return_type_of_t<F, error_type&>>
-    map_err(F&& f) &
-    {
-        if(this->is_err()){return err(f(this->as_err()));}
-        return ok(this->as_ok());
-    }
-    template<typename F>
-    result<value_type, detail::return_type_of_t<F, error_type const&>>
-    map_err(F&& f) const&
-    {
-        if(this->is_err()){return err(f(this->as_err()));}
-        return ok(this->as_ok());
-    }
-    template<typename F>
-    result<value_type, detail::return_type_of_t<F, error_type&&>>
-    map_err(F&& f) &&
-    {
-        if(this->is_err()){return err(f(std::move(this->as_err())));}
-        return ok(std::move(this->as_ok()));
-    }
-
-    // prerequisities
-    // F: T -> U
-    // retval: U
-    template<typename F, typename U>
-    detail::return_type_of_t<F, value_type&>
-    map_or_else(F&& f, U&& opt) &
-    {
-        if(this->is_err()){return std::forward<U>(opt);}
-        return f(this->as_ok());
-    }
-    template<typename F, typename U>
-    detail::return_type_of_t<F, value_type const&>
-    map_or_else(F&& f, U&& opt) const&
-    {
-        if(this->is_err()){return std::forward<U>(opt);}
-        return f(this->as_ok());
-    }
-    template<typename F, typename U>
-    detail::return_type_of_t<F, value_type&&>
-    map_or_else(F&& f, U&& opt) &&
-    {
-        if(this->is_err()){return std::forward<U>(opt);}
-        return f(std::move(this->as_ok()));
-    }
-
-    // prerequisities
-    // F: E -> U
-    // retval: U
-    template<typename F, typename U>
-    detail::return_type_of_t<F, error_type&>
-    map_err_or_else(F&& f, U&& opt) &
-    {
-        if(this->is_ok()){return std::forward<U>(opt);}
-        return f(this->as_err());
-    }
-    template<typename F, typename U>
-    detail::return_type_of_t<F, error_type const&>
-    map_err_or_else(F&& f, U&& opt) const&
-    {
-        if(this->is_ok()){return std::forward<U>(opt);}
-        return f(this->as_err());
-    }
-    template<typename F, typename U>
-    detail::return_type_of_t<F, error_type&&>
-    map_err_or_else(F&& f, U&& opt) &&
-    {
-        if(this->is_ok()){return std::forward<U>(opt);}
-        return f(std::move(this->as_err()));
-    }
-
-    // prerequisities:
-    // F: func T -> U
-    // toml::err(error_type) should be convertible to U.
-    // normally, type U is another result<S, F> and E is convertible to F
-    template<typename F>
-    detail::return_type_of_t<F, value_type&>
-    and_then(F&& f) &
-    {
-        if(this->is_ok()){return f(this->as_ok());}
-        return err(this->as_err());
-    }
-    template<typename F>
-    detail::return_type_of_t<F, value_type const&>
-    and_then(F&& f) const&
-    {
-        if(this->is_ok()){return f(this->as_ok());}
-        return err(this->as_err());
-    }
-    template<typename F>
-    detail::return_type_of_t<F, value_type&&>
-    and_then(F&& f) &&
-    {
-        if(this->is_ok()){return f(std::move(this->as_ok()));}
-        return err(std::move(this->as_err()));
-    }
-
-    // prerequisities:
-    // F: func E -> U
-    // toml::ok(value_type) should be convertible to U.
-    // normally, type U is another result<S, F> and T is convertible to S
-    template<typename F>
-    detail::return_type_of_t<F, error_type&>
-    or_else(F&& f) &
-    {
-        if(this->is_err()){return f(this->as_err());}
-        return ok(this->as_ok());
-    }
-    template<typename F>
-    detail::return_type_of_t<F, error_type const&>
-    or_else(F&& f) const&
-    {
-        if(this->is_err()){return f(this->as_err());}
-        return ok(this->as_ok());
-    }
-    template<typename F>
-    detail::return_type_of_t<F, error_type&&>
-    or_else(F&& f) &&
-    {
-        if(this->is_err()){return f(std::move(this->as_err()));}
-        return ok(std::move(this->as_ok()));
-    }
-
-    // if *this is error, returns *this. otherwise, returns other.
-    result and_other(const result& other) const&
-    {
-        return this->is_err() ? *this : other;
-    }
-    result and_other(result&& other) &&
-    {
-        return this->is_err() ? std::move(*this) : std::move(other);
-    }
-
-    // if *this is okay, returns *this. otherwise, returns other.
-    result or_other(const result& other) const&
-    {
-        return this->is_ok() ? *this : other;
-    }
-    result or_other(result&& other) &&
-    {
-        return this->is_ok() ? std::move(*this) : std::move(other);
-    }
-
-    void swap(result<T, E>& other)
-    {
-        result<T, E> tmp(std::move(*this));
-        *this = std::move(other);
-        other = std::move(tmp);
-        return ;
-    }
-
-  private:
-
-    static std::string format_error(std::exception const& excpt)
-    {
-        return std::string(excpt.what());
-    }
-    template<typename U, typename std::enable_if<!std::is_base_of<
-        std::exception, U>::value, std::nullptr_t>::type = nullptr>
-    static std::string format_error(U const& others)
-    {
-        std::ostringstream oss; oss << others;
-        return oss.str();
-    }
-
-    void cleanup() noexcept
-    {
-        if(this->is_ok_) {this->succ.~success_type();}
-        else             {this->fail.~failure_type();}
-        return;
-    }
-
-  private:
-
-    bool      is_ok_;
-    union
-    {
-        success_type succ;
-        failure_type fail;
-    };
-};
-
-template<typename T, typename E>
-void swap(result<T, E>& lhs, result<T, E>& rhs)
-{
-    lhs.swap(rhs);
-    return;
-}
-
-// this might be confusing because it eagerly evaluated, while in the other
-// cases operator && and || are short-circuited.
-//
-// template<typename T, typename E>
-// inline result<T, E>
-// operator&&(const result<T, E>& lhs, const result<T, E>& rhs) noexcept
-// {
-//     return lhs.is_ok() ? rhs : lhs;
-// }
-//
-// template<typename T, typename E>
-// inline result<T, E>
-// operator||(const result<T, E>& lhs, const result<T, E>& rhs) noexcept
-// {
-//     return lhs.is_ok() ? lhs : rhs;
-// }
-
-// ----------------------------------------------------------------------------
-// re-use result<T, E> as a optional<T> with none_t
-
-namespace detail
-{
-struct none_t {};
-inline bool operator==(const none_t&, const none_t&) noexcept {return true;}
-inline bool operator!=(const none_t&, const none_t&) noexcept {return false;}
-inline bool operator< (const none_t&, const none_t&) noexcept {return false;}
-inline bool operator<=(const none_t&, const none_t&) noexcept {return true;}
-inline bool operator> (const none_t&, const none_t&) noexcept {return false;}
-inline bool operator>=(const none_t&, const none_t&) noexcept {return true;}
-template<typename charT, typename traitsT>
-std::basic_ostream<charT, traitsT>&
-operator<<(std::basic_ostream<charT, traitsT>& os, const none_t&)
-{
-    os << "none";
-    return os;
-}
-inline failure<none_t> none() noexcept {return failure<none_t>{none_t{}};}
-} // detail
-} // toml11
-#endif// TOML11_RESULT_H
diff --git a/src/frontend/qt_sdl/toml/toml/serializer.hpp b/src/frontend/qt_sdl/toml/toml/serializer.hpp
deleted file mode 100644
index 88ae775a..00000000
--- a/src/frontend/qt_sdl/toml/toml/serializer.hpp
+++ /dev/null
@@ -1,922 +0,0 @@
-//     Copyright Toru Niina 2019.
-// Distributed under the MIT License.
-#ifndef TOML11_SERIALIZER_HPP
-#define TOML11_SERIALIZER_HPP
-#include <cmath>
-#include <cstdio>
-
-#include <limits>
-
-#include "lexer.hpp"
-#include "value.hpp"
-
-namespace toml
-{
-
-// This function serialize a key. It checks a string is a bare key and
-// escapes special characters if the string is not compatible to a bare key.
-// ```cpp
-// std::string k("non.bare.key"); // the key itself includes `.`s.
-// std::string formatted = toml::format_key(k);
-// assert(formatted == "\"non.bare.key\"");
-// ```
-//
-// This function is exposed to make it easy to write a user-defined serializer.
-// Since toml restricts characters available in a bare key, generally a string
-// should be escaped. But checking whether a string needs to be surrounded by
-// a `"` and escaping some special character is boring.
-template<typename charT, typename traits, typename Alloc>
-std::basic_string<charT, traits, Alloc>
-format_key(const std::basic_string<charT, traits, Alloc>& k)
-{
-    if(k.empty())
-    {
-        return std::string("\"\"");
-    }
-
-    // check the key can be a bare (unquoted) key
-    detail::location loc(k, std::vector<char>(k.begin(), k.end()));
-    detail::lex_unquoted_key::invoke(loc);
-    if(loc.iter() == loc.end())
-    {
-        return k; // all the tokens are consumed. the key is unquoted-key.
-    }
-
-    //if it includes special characters, then format it in a "quoted" key.
-    std::basic_string<charT, traits, Alloc> serialized("\"");
-    for(const char c : k)
-    {
-        switch(c)
-        {
-            case '\\': {serialized += "\\\\"; break;}
-            case '\"': {serialized += "\\\""; break;}
-            case '\b': {serialized += "\\b";  break;}
-            case '\t': {serialized += "\\t";  break;}
-            case '\f': {serialized += "\\f";  break;}
-            case '\n': {serialized += "\\n";  break;}
-            case '\r': {serialized += "\\r";  break;}
-            default  : {serialized += c;      break;}
-        }
-    }
-    serialized += "\"";
-    return serialized;
-}
-
-template<typename charT, typename traits, typename Alloc>
-std::basic_string<charT, traits, Alloc>
-format_keys(const std::vector<std::basic_string<charT, traits, Alloc>>& keys)
-{
-    if(keys.empty())
-    {
-        return std::string("\"\"");
-    }
-
-    std::basic_string<charT, traits, Alloc> serialized;
-    for(const auto& ky : keys)
-    {
-        serialized += format_key(ky);
-        serialized += charT('.');
-    }
-    serialized.pop_back(); // remove the last dot '.'
-    return serialized;
-}
-
-template<typename Value>
-struct serializer
-{
-    static_assert(detail::is_basic_value<Value>::value,
-                  "toml::serializer is for toml::value and its variants, "
-                  "toml::basic_value<...>.");
-
-    using value_type           = Value;
-    using key_type             = typename value_type::key_type            ;
-    using comment_type         = typename value_type::comment_type        ;
-    using boolean_type         = typename value_type::boolean_type        ;
-    using integer_type         = typename value_type::integer_type        ;
-    using floating_type        = typename value_type::floating_type       ;
-    using string_type          = typename value_type::string_type         ;
-    using local_time_type      = typename value_type::local_time_type     ;
-    using local_date_type      = typename value_type::local_date_type     ;
-    using local_datetime_type  = typename value_type::local_datetime_type ;
-    using offset_datetime_type = typename value_type::offset_datetime_type;
-    using array_type           = typename value_type::array_type          ;
-    using table_type           = typename value_type::table_type          ;
-
-    serializer(const std::size_t w              = 80u,
-               const int         float_prec     = std::numeric_limits<toml::floating>::max_digits10,
-               const bool        can_be_inlined = false,
-               const bool        no_comment     = false,
-               std::vector<toml::key> ks        = {},
-               const bool     value_has_comment = false)
-        : can_be_inlined_(can_be_inlined), no_comment_(no_comment),
-          value_has_comment_(value_has_comment && !no_comment),
-          float_prec_(float_prec), width_(w), keys_(std::move(ks))
-    {}
-    ~serializer() = default;
-
-    std::string operator()(const boolean_type& b) const
-    {
-        return b ? "true" : "false";
-    }
-    std::string operator()(const integer_type i) const
-    {
-        return std::to_string(i);
-    }
-    std::string operator()(const floating_type f) const
-    {
-        if(std::isnan(f))
-        {
-            if(std::signbit(f))
-            {
-                return std::string("-nan");
-            }
-            else
-            {
-                return std::string("nan");
-            }
-        }
-        else if(!std::isfinite(f))
-        {
-            if(std::signbit(f))
-            {
-                return std::string("-inf");
-            }
-            else
-            {
-                return std::string("inf");
-            }
-        }
-
-        const auto fmt = "%.*g";
-        const auto bsz = std::snprintf(nullptr, 0, fmt, this->float_prec_, f);
-        // +1 for null character(\0)
-        std::vector<char> buf(static_cast<std::size_t>(bsz + 1), '\0');
-        std::snprintf(buf.data(), buf.size(), fmt, this->float_prec_, f);
-
-        std::string token(buf.begin(), std::prev(buf.end()));
-        if(!token.empty() && token.back() == '.') // 1. => 1.0
-        {
-            token += '0';
-        }
-
-        const auto e = std::find_if(
-            token.cbegin(), token.cend(), [](const char c) noexcept -> bool {
-                return c == 'e' || c == 'E';
-            });
-        const auto has_exponent = (token.cend() != e);
-        const auto has_fraction = (token.cend() != std::find(
-            token.cbegin(), token.cend(), '.'));
-
-        if(!has_exponent && !has_fraction)
-        {
-            // the resulting value does not have any float specific part!
-            token += ".0";
-        }
-        return token;
-    }
-    std::string operator()(const string_type& s) const
-    {
-        if(s.kind == string_t::basic)
-        {
-            if((std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend() ||
-                std::find(s.str.cbegin(), s.str.cend(), '\"') != s.str.cend()) &&
-               this->width_ != (std::numeric_limits<std::size_t>::max)())
-            {
-                // if linefeed or double-quote is contained,
-                // make it multiline basic string.
-                const auto escaped = this->escape_ml_basic_string(s.str);
-                std::string open("\"\"\"");
-                std::string close("\"\"\"");
-                if(escaped.find('\n') != std::string::npos ||
-                   this->width_ < escaped.size() + 6)
-                {
-                    // if the string body contains newline or is enough long,
-                    // add newlines after and before delimiters.
-                    open += "\n";
-                    close = std::string("\\\n") + close;
-                }
-                return open + escaped + close;
-            }
-
-            // no linefeed. try to make it oneline-string.
-            std::string oneline = this->escape_basic_string(s.str);
-            if(oneline.size() + 2 < width_ || width_ < 2)
-            {
-                const std::string quote("\"");
-                return quote + oneline + quote;
-            }
-
-            // the line is too long compared to the specified width.
-            // split it into multiple lines.
-            std::string token("\"\"\"\n");
-            while(!oneline.empty())
-            {
-                if(oneline.size() < width_)
-                {
-                    token += oneline;
-                    oneline.clear();
-                }
-                else if(oneline.at(width_-2) == '\\')
-                {
-                    token += oneline.substr(0, width_-2);
-                    token += "\\\n";
-                    oneline.erase(0, width_-2);
-                }
-                else
-                {
-                    token += oneline.substr(0, width_-1);
-                    token += "\\\n";
-                    oneline.erase(0, width_-1);
-                }
-            }
-            return token + std::string("\\\n\"\"\"");
-        }
-        else // the string `s` is literal-string.
-        {
-            if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend() ||
-               std::find(s.str.cbegin(), s.str.cend(), '\'') != s.str.cend() )
-            {
-                std::string open("'''");
-                if(this->width_ + 6 < s.str.size())
-                {
-                    open += '\n'; // the first newline is ignored by TOML spec
-                }
-                const std::string close("'''");
-                return open + s.str + close;
-            }
-            else
-            {
-                const std::string quote("'");
-                return quote + s.str + quote;
-            }
-        }
-    }
-
-    std::string operator()(const local_date_type& d) const
-    {
-        std::ostringstream oss;
-        oss << d;
-        return oss.str();
-    }
-    std::string operator()(const local_time_type& t) const
-    {
-        std::ostringstream oss;
-        oss << t;
-        return oss.str();
-    }
-    std::string operator()(const local_datetime_type& dt) const
-    {
-        std::ostringstream oss;
-        oss << dt;
-        return oss.str();
-    }
-    std::string operator()(const offset_datetime_type& odt) const
-    {
-        std::ostringstream oss;
-        oss << odt;
-        return oss.str();
-    }
-
-    std::string operator()(const array_type& v) const
-    {
-        if(v.empty())
-        {
-            return std::string("[]");
-        }
-        if(this->is_array_of_tables(v))
-        {
-            return make_array_of_tables(v);
-        }
-
-        // not an array of tables. normal array.
-        // first, try to make it inline if none of the elements have a comment.
-        if( ! this->has_comment_inside(v))
-        {
-            const auto inl = this->make_inline_array(v);
-            if(inl.size() < this->width_ &&
-               std::find(inl.cbegin(), inl.cend(), '\n') == inl.cend())
-            {
-                return inl;
-            }
-        }
-
-        // if the length exceeds this->width_, print multiline array.
-        // key = [
-        //   # ...
-        //   42,
-        //   ...
-        // ]
-        std::string token;
-        std::string current_line;
-        token += "[\n";
-        for(const auto& item : v)
-        {
-            if( ! item.comments().empty() && !no_comment_)
-            {
-                // if comment exists, the element must be the only element in the line.
-                // e.g. the following is not allowed.
-                // ```toml
-                // array = [
-                // # comment for what?
-                // 1, 2, 3, 4, 5
-                // ]
-                // ```
-                if(!current_line.empty())
-                {
-                    if(current_line.back() != '\n')
-                    {
-                        current_line += '\n';
-                    }
-                    token += current_line;
-                    current_line.clear();
-                }
-                for(const auto& c : item.comments())
-                {
-                    token += '#';
-                    token += c;
-                    token += '\n';
-                }
-                token += toml::visit(*this, item);
-                if(!token.empty() && token.back() == '\n') {token.pop_back();}
-                token += ",\n";
-                continue;
-            }
-            std::string next_elem;
-            if(item.is_table())
-            {
-                serializer ser(*this);
-                ser.can_be_inlined_ = true;
-                ser.width_ = (std::numeric_limits<std::size_t>::max)();
-                next_elem += toml::visit(ser, item);
-            }
-            else
-            {
-                next_elem += toml::visit(*this, item);
-            }
-
-            // comma before newline.
-            if(!next_elem.empty() && next_elem.back() == '\n') {next_elem.pop_back();}
-
-            // if current line does not exceeds the width limit, continue.
-            if(current_line.size() + next_elem.size() + 1 < this->width_)
-            {
-                current_line += next_elem;
-                current_line += ',';
-            }
-            else if(current_line.empty())
-            {
-                // if current line was empty, force put the next_elem because
-                // next_elem is not splittable
-                token += next_elem;
-                token += ",\n";
-                // current_line is kept empty
-            }
-            else // reset current_line
-            {
-                assert(current_line.back() == ',');
-                token += current_line;
-                token += '\n';
-                current_line = next_elem;
-                current_line += ',';
-            }
-        }
-        if(!current_line.empty())
-        {
-            if(!current_line.empty() && current_line.back() != '\n')
-            {
-                current_line += '\n';
-            }
-            token += current_line;
-        }
-        token += "]\n";
-        return token;
-    }
-
-    // templatize for any table-like container
-    std::string operator()(const table_type& v) const
-    {
-        // if an element has a comment, then it can't be inlined.
-        // table = {# how can we write a comment for this? key = "value"}
-        if(this->can_be_inlined_ && !(this->has_comment_inside(v)))
-        {
-            std::string token;
-            if(!this->keys_.empty())
-            {
-                token += format_key(this->keys_.back());
-                token += " = ";
-            }
-            token += this->make_inline_table(v);
-            if(token.size() < this->width_ &&
-               token.end() == std::find(token.begin(), token.end(), '\n'))
-            {
-                return token;
-            }
-        }
-
-        std::string token;
-        if(!keys_.empty())
-        {
-            token += '[';
-            token += format_keys(keys_);
-            token += "]\n";
-        }
-        token += this->make_multiline_table(v);
-        return token;
-    }
-
-  private:
-
-    std::string escape_basic_string(const std::string& s) const
-    {
-        //XXX assuming `s` is a valid utf-8 sequence.
-        std::string retval;
-        for(const char c : s)
-        {
-            switch(c)
-            {
-                case '\\': {retval += "\\\\"; break;}
-                case '\"': {retval += "\\\""; break;}
-                case '\b': {retval += "\\b";  break;}
-                case '\t': {retval += "\\t";  break;}
-                case '\f': {retval += "\\f";  break;}
-                case '\n': {retval += "\\n";  break;}
-                case '\r': {retval += "\\r";  break;}
-                default  :
-                {
-                    if((0x00 <= c && c <= 0x08) || (0x0A <= c && c <= 0x1F) || c == 0x7F)
-                    {
-                        retval += "\\u00";
-                        retval += char(48 + (c / 16));
-                        retval += char((c % 16 < 10 ? 48 : 55) + (c % 16));
-                    }
-                    else
-                    {
-                        retval += c;
-                    }
-                }
-            }
-        }
-        return retval;
-    }
-
-    std::string escape_ml_basic_string(const std::string& s) const
-    {
-        std::string retval;
-        for(auto i=s.cbegin(), e=s.cend(); i!=e; ++i)
-        {
-            switch(*i)
-            {
-                case '\\': {retval += "\\\\"; break;}
-                // One or two consecutive "s are allowed.
-                // Later we will check there are no three consecutive "s.
-                //   case '\"': {retval += "\\\""; break;}
-                case '\b': {retval += "\\b";  break;}
-                case '\t': {retval += "\\t";  break;}
-                case '\f': {retval += "\\f";  break;}
-                case '\n': {retval += "\n";   break;}
-                case '\r':
-                {
-                    if(std::next(i) != e && *std::next(i) == '\n')
-                    {
-                        retval += "\r\n";
-                        ++i;
-                    }
-                    else
-                    {
-                        retval += "\\r";
-                    }
-                    break;
-                }
-                default  :
-                {
-                    const auto c = *i;
-                    if((0x00 <= c && c <= 0x08) || (0x0A <= c && c <= 0x1F) || c == 0x7F)
-                    {
-                        retval += "\\u00";
-                        retval += char(48 + (c / 16));
-                        retval += char((c % 16 < 10 ? 48 : 55) + (c % 16));
-                    }
-                    else
-                    {
-                        retval += c;
-                    }
-                }
-
-            }
-        }
-        // Only 1 or 2 consecutive `"`s are allowed in multiline basic string.
-        // 3 consecutive `"`s are considered as a closing delimiter.
-        // We need to check if there are 3 or more consecutive `"`s and insert
-        // backslash to break them down into several short `"`s like the `str6`
-        // in the following example.
-        // ```toml
-        // str4 = """Here are two quotation marks: "". Simple enough."""
-        // # str5 = """Here are three quotation marks: """."""  # INVALID
-        // str5 = """Here are three quotation marks: ""\"."""
-        // str6 = """Here are fifteen quotation marks: ""\"""\"""\"""\"""\"."""
-        // ```
-        auto found_3_quotes = retval.find("\"\"\"");
-        while(found_3_quotes != std::string::npos)
-        {
-            retval.replace(found_3_quotes, 3, "\"\"\\\"");
-            found_3_quotes = retval.find("\"\"\"");
-        }
-        return retval;
-    }
-
-    // if an element of a table or an array has a comment, it cannot be inlined.
-    bool has_comment_inside(const array_type& a) const noexcept
-    {
-        // if no_comment is set, comments would not be written.
-        if(this->no_comment_) {return false;}
-
-        for(const auto& v : a)
-        {
-            if(!v.comments().empty()) {return true;}
-        }
-        return false;
-    }
-    bool has_comment_inside(const table_type& t) const noexcept
-    {
-        // if no_comment is set, comments would not be written.
-        if(this->no_comment_) {return false;}
-
-        for(const auto& kv : t)
-        {
-            if(!kv.second.comments().empty()) {return true;}
-        }
-        return false;
-    }
-
-    std::string make_inline_array(const array_type& v) const
-    {
-        assert(!has_comment_inside(v));
-        std::string token;
-        token += '[';
-        bool is_first = true;
-        for(const auto& item : v)
-        {
-            if(is_first) {is_first = false;} else {token += ',';}
-            token += visit(serializer(
-                (std::numeric_limits<std::size_t>::max)(), this->float_prec_,
-                /* inlined */ true, /*no comment*/ false, /*keys*/ {},
-                /*has_comment*/ !item.comments().empty()), item);
-        }
-        token += ']';
-        return token;
-    }
-
-    std::string make_inline_table(const table_type& v) const
-    {
-        assert(!has_comment_inside(v));
-        assert(this->can_be_inlined_);
-        std::string token;
-        token += '{';
-        bool is_first = true;
-        for(const auto& kv : v)
-        {
-            // in inline tables, trailing comma is not allowed (toml-lang #569).
-            if(is_first) {is_first = false;} else {token += ',';}
-            token += format_key(kv.first);
-            token += '=';
-            token += visit(serializer(
-                (std::numeric_limits<std::size_t>::max)(), this->float_prec_,
-                /* inlined */ true, /*no comment*/ false, /*keys*/ {},
-                /*has_comment*/ !kv.second.comments().empty()), kv.second);
-        }
-        token += '}';
-        return token;
-    }
-
-    std::string make_multiline_table(const table_type& v) const
-    {
-        std::string token;
-
-        // print non-table elements first.
-        // ```toml
-        // [foo]         # a table we're writing now here
-        // key = "value" # <- non-table element, "key"
-        // # ...
-        // [foo.bar] # <- table element, "bar"
-        // ```
-        // because after printing [foo.bar], the remaining non-table values will
-        // be assigned into [foo.bar], not [foo]. Those values should be printed
-        // earlier.
-        for(const auto& kv : v)
-        {
-            if(kv.second.is_table() || is_array_of_tables(kv.second))
-            {
-                continue;
-            }
-
-            token += write_comments(kv.second);
-
-            const auto key_and_sep    = format_key(kv.first) + " = ";
-            const auto residual_width = (this->width_ > key_and_sep.size()) ?
-                                        this->width_ - key_and_sep.size() : 0;
-            token += key_and_sep;
-            token += visit(serializer(residual_width, this->float_prec_,
-                /*can be inlined*/ true, /*no comment*/ false, /*keys*/ {},
-                /*has_comment*/ !kv.second.comments().empty()), kv.second);
-
-            if(token.back() != '\n')
-            {
-                token += '\n';
-            }
-        }
-
-        // normal tables / array of tables
-
-        // after multiline table appeared, the other tables cannot be inline
-        // because the table would be assigned into the table.
-        // [foo]
-        // ...
-        // bar = {...} # <- bar will be a member of [foo].
-        bool multiline_table_printed = false;
-        for(const auto& kv : v)
-        {
-            if(!kv.second.is_table() && !is_array_of_tables(kv.second))
-            {
-                continue; // other stuff are already serialized. skip them.
-            }
-
-            std::vector<toml::key> ks(this->keys_);
-            ks.push_back(kv.first);
-
-            auto tmp = visit(serializer(this->width_, this->float_prec_,
-                !multiline_table_printed, this->no_comment_, ks,
-                /*has_comment*/ !kv.second.comments().empty()), kv.second);
-
-            // If it is the first time to print a multi-line table, it would be
-            // helpful to separate normal key-value pair and subtables by a
-            // newline.
-            // (this checks if the current key-value pair contains newlines.
-            //  but it is not perfect because multi-line string can also contain
-            //  a newline. in such a case, an empty line will be written) TODO
-            if((!multiline_table_printed) &&
-               std::find(tmp.cbegin(), tmp.cend(), '\n') != tmp.cend())
-            {
-                multiline_table_printed = true;
-                token += '\n'; // separate key-value pairs and subtables
-
-                token += write_comments(kv.second);
-                token += tmp;
-
-                // care about recursive tables (all tables in each level prints
-                // newline and there will be a full of newlines)
-                if(tmp.substr(tmp.size() - 2, 2) != "\n\n" &&
-                   tmp.substr(tmp.size() - 4, 4) != "\r\n\r\n" )
-                {
-                    token += '\n';
-                }
-            }
-            else
-            {
-                token += write_comments(kv.second);
-                token += tmp;
-                token += '\n';
-            }
-        }
-        return token;
-    }
-
-    std::string make_array_of_tables(const array_type& v) const
-    {
-        // if it's not inlined, we need to add `[[table.key]]`.
-        // but if it can be inlined, we can format it as the following.
-        // ```
-        // table.key = [
-        //   {...},
-        //   # comment
-        //   {...},
-        // ]
-        // ```
-        // This function checks if inlinization is possible or not, and then
-        // format the array-of-tables in a proper way.
-        //
-        // Note about comments:
-        //
-        // If the array itself has a comment (value_has_comment_ == true), we
-        // should try to make it inline.
-        // ```toml
-        // # comment about array
-        // array = [
-        //   # comment about table element
-        //   {of = "table"}
-        // ]
-        // ```
-        // If it is formatted as a multiline table, the two comments becomes
-        // indistinguishable.
-        // ```toml
-        // # comment about array
-        // # comment about table element
-        // [[array]]
-        // of = "table"
-        // ```
-        // So we need to try to make it inline, and it force-inlines regardless
-        // of the line width limit.
-        //     It may fail if the element of a table has comment. In that case,
-        // the array-of-tables will be formatted as a multiline table.
-        if(this->can_be_inlined_ || this->value_has_comment_)
-        {
-            std::string token;
-            if(!keys_.empty())
-            {
-                token += format_key(keys_.back());
-                token += " = ";
-            }
-
-            bool failed = false;
-            token += "[\n";
-            for(const auto& item : v)
-            {
-                // if an element of the table has a comment, the table
-                // cannot be inlined.
-                if(this->has_comment_inside(item.as_table()))
-                {
-                    failed = true;
-                    break;
-                }
-                // write comments for the table itself
-                token += write_comments(item);
-
-                const auto t = this->make_inline_table(item.as_table());
-
-                if(t.size() + 1 > width_ || // +1 for the last comma {...},
-                   std::find(t.cbegin(), t.cend(), '\n') != t.cend())
-                {
-                    // if the value itself has a comment, ignore the line width limit
-                    if( ! this->value_has_comment_)
-                    {
-                        failed = true;
-                        break;
-                    }
-                }
-                token += t;
-                token += ",\n";
-            }
-
-            if( ! failed)
-            {
-                token += "]\n";
-                return token;
-            }
-            // if failed, serialize them as [[array.of.tables]].
-        }
-
-        std::string token;
-        for(const auto& item : v)
-        {
-            token += write_comments(item);
-            token += "[[";
-            token += format_keys(keys_);
-            token += "]]\n";
-            token += this->make_multiline_table(item.as_table());
-        }
-        return token;
-    }
-
-    std::string write_comments(const value_type& v) const
-    {
-        std::string retval;
-        if(this->no_comment_) {return retval;}
-
-        for(const auto& c : v.comments())
-        {
-            retval += '#';
-            retval += c;
-            retval += '\n';
-        }
-        return retval;
-    }
-
-    bool is_array_of_tables(const value_type& v) const
-    {
-        if(!v.is_array() || v.as_array().empty()) {return false;}
-        return is_array_of_tables(v.as_array());
-    }
-    bool is_array_of_tables(const array_type& v) const
-    {
-        // Since TOML v0.5.0, heterogeneous arrays are allowed. So we need to
-        // check all the element in an array to check if the array is an array
-        // of tables.
-        return std::all_of(v.begin(), v.end(), [](const value_type& elem) {
-                return elem.is_table();
-            });
-    }
-
-  private:
-
-    bool        can_be_inlined_;
-    bool        no_comment_;
-    bool        value_has_comment_;
-    int         float_prec_;
-    std::size_t width_;
-    std::vector<toml::key> keys_;
-};
-
-template<typename C,
-         template<typename ...> class M, template<typename ...> class V>
-std::string
-format(const basic_value<C, M, V>& v, std::size_t w = 80u,
-       int fprec = std::numeric_limits<toml::floating>::max_digits10,
-       bool no_comment = false, bool force_inline = false)
-{
-    using value_type = basic_value<C, M, V>;
-    // if value is a table, it is considered to be a root object.
-    // the root object can't be an inline table.
-    if(v.is_table())
-    {
-        std::ostringstream oss;
-        if(!v.comments().empty())
-        {
-            oss << v.comments();
-            oss << '\n'; // to split the file comment from the first element
-        }
-        const auto serialized = visit(serializer<value_type>(w, fprec, false, no_comment), v);
-        oss << serialized;
-        return oss.str();
-    }
-    return visit(serializer<value_type>(w, fprec, force_inline), v);
-}
-
-namespace detail
-{
-template<typename charT, typename traits>
-int comment_index(std::basic_ostream<charT, traits>&)
-{
-    static const int index = std::ios_base::xalloc();
-    return index;
-}
-} // detail
-
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-nocomment(std::basic_ostream<charT, traits>& os)
-{
-    // by default, it is zero. and by default, it shows comments.
-    os.iword(detail::comment_index(os)) = 1;
-    return os;
-}
-
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-showcomment(std::basic_ostream<charT, traits>& os)
-{
-    // by default, it is zero. and by default, it shows comments.
-    os.iword(detail::comment_index(os)) = 0;
-    return os;
-}
-
-template<typename charT, typename traits, typename C,
-         template<typename ...> class M, template<typename ...> class V>
-std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, const basic_value<C, M, V>& v)
-{
-    using value_type = basic_value<C, M, V>;
-
-    // get status of std::setw().
-    const auto w     = static_cast<std::size_t>(os.width());
-    const int  fprec = static_cast<int>(os.precision());
-    os.width(0);
-
-    // by default, iword is initialized by 0. And by default, toml11 outputs
-    // comments. So `0` means showcomment. 1 means nocommnet.
-    const bool no_comment = (1 == os.iword(detail::comment_index(os)));
-
-    if(!no_comment && v.is_table() && !v.comments().empty())
-    {
-        os << v.comments();
-        os << '\n'; // to split the file comment from the first element
-    }
-    // the root object can't be an inline table. so pass `false`.
-    const auto serialized = visit(serializer<value_type>(w, fprec, no_comment, false), v);
-    os << serialized;
-
-    // if v is a non-table value, and has only one comment, then
-    // put a comment just after a value. in the following way.
-    //
-    // ```toml
-    // key = "value" # comment.
-    // ```
-    //
-    // Since the top-level toml object is a table, one who want to put a
-    // non-table toml value must use this in a following way.
-    //
-    // ```cpp
-    // toml::value v;
-    // std::cout << "user-defined-key = " << v << std::endl;
-    // ```
-    //
-    // In this case, it is impossible to put comments before key-value pair.
-    // The only way to preserve comments is to put all of them after a value.
-    if(!no_comment && !v.is_table() && !v.comments().empty())
-    {
-        os << " #";
-        for(const auto& c : v.comments()) {os << c;}
-    }
-    return os;
-}
-
-} // toml
-#endif// TOML11_SERIALIZER_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/source_location.hpp b/src/frontend/qt_sdl/toml/toml/source_location.hpp
deleted file mode 100644
index fa175b5b..00000000
--- a/src/frontend/qt_sdl/toml/toml/source_location.hpp
+++ /dev/null
@@ -1,233 +0,0 @@
-//     Copyright Toru Niina 2019.
-// Distributed under the MIT License.
-#ifndef TOML11_SOURCE_LOCATION_HPP
-#define TOML11_SOURCE_LOCATION_HPP
-#include <cstdint>
-#include <sstream>
-
-#include "region.hpp"
-
-namespace toml
-{
-
-// A struct to contain location in a toml file.
-// The interface imitates std::experimental::source_location,
-// but not completely the same.
-//
-// It would be constructed by toml::value. It can be used to generate
-// user-defined error messages.
-//
-// - std::uint_least32_t line() const noexcept
-//   - returns the line number where the region is on.
-// - std::uint_least32_t column() const noexcept
-//   - returns the column number where the region starts.
-// - std::uint_least32_t region() const noexcept
-//   - returns the size of the region.
-//
-// +-- line()       +-- region of interest (region() == 9)
-// v            .---+---.
-// 12 | value = "foo bar"
-//              ^
-//              +-- column()
-//
-// - std::string const& file_name() const noexcept;
-//   - name of the file.
-// - std::string const& line_str() const noexcept;
-//   - the whole line that contains the region of interest.
-//
-struct source_location
-{
-  public:
-
-    source_location()
-        : line_num_(1), column_num_(1), region_size_(1),
-          file_name_("unknown file"), line_str_("")
-    {}
-
-    explicit source_location(const detail::region_base* reg)
-        : line_num_(1), column_num_(1), region_size_(1),
-          file_name_("unknown file"), line_str_("")
-    {
-        if(reg)
-        {
-            if(reg->line_num() != detail::region_base().line_num())
-            {
-                line_num_ = static_cast<std::uint_least32_t>(
-                        std::stoul(reg->line_num()));
-            }
-            column_num_  = static_cast<std::uint_least32_t>(reg->before() + 1);
-            region_size_ = static_cast<std::uint_least32_t>(reg->size());
-            file_name_   = reg->name();
-            line_str_    = reg->line();
-        }
-    }
-
-    explicit source_location(const detail::region& reg)
-        : line_num_(static_cast<std::uint_least32_t>(std::stoul(reg.line_num()))),
-          column_num_(static_cast<std::uint_least32_t>(reg.before() + 1)),
-          region_size_(static_cast<std::uint_least32_t>(reg.size())),
-          file_name_(reg.name()),
-          line_str_ (reg.line())
-    {}
-    explicit source_location(const detail::location& loc)
-        : line_num_(static_cast<std::uint_least32_t>(std::stoul(loc.line_num()))),
-          column_num_(static_cast<std::uint_least32_t>(loc.before() + 1)),
-          region_size_(static_cast<std::uint_least32_t>(loc.size())),
-          file_name_(loc.name()),
-          line_str_ (loc.line())
-    {}
-
-    ~source_location() = default;
-    source_location(source_location const&) = default;
-    source_location(source_location &&)     = default;
-    source_location& operator=(source_location const&) = default;
-    source_location& operator=(source_location &&)     = default;
-
-    std::uint_least32_t line()      const noexcept {return line_num_;}
-    std::uint_least32_t column()    const noexcept {return column_num_;}
-    std::uint_least32_t region()    const noexcept {return region_size_;}
-
-    std::string const&  file_name() const noexcept {return file_name_;}
-    std::string const&  line_str()  const noexcept {return line_str_;}
-
-  private:
-
-    std::uint_least32_t line_num_;
-    std::uint_least32_t column_num_;
-    std::uint_least32_t region_size_;
-    std::string         file_name_;
-    std::string         line_str_;
-};
-
-namespace detail
-{
-
-// internal error message generation.
-inline std::string format_underline(const std::string& message,
-        const std::vector<std::pair<source_location, std::string>>& loc_com,
-        const std::vector<std::string>& helps = {},
-        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
-{
-    std::size_t line_num_width = 0;
-    for(const auto& lc : loc_com)
-    {
-        std::uint_least32_t line = lc.first.line();
-        std::size_t        digit = 0;
-        while(line != 0)
-        {
-            line  /= 10;
-            digit +=  1;
-        }
-        line_num_width = (std::max)(line_num_width, digit);
-    }
-    // 1 is the minimum width
-    line_num_width = std::max<std::size_t>(line_num_width, 1);
-
-    std::ostringstream retval;
-
-    if(colorize)
-    {
-        retval << color::colorize; // turn on ANSI color
-    }
-
-    // XXX
-    // Here, before `colorize` support, it does not output `[error]` prefix
-    // automatically. So some user may output it manually and this change may
-    // duplicate the prefix. To avoid it, check the first 7 characters and
-    // if it is "[error]", it removes that part from the message shown.
-    if(message.size() > 7 && message.substr(0, 7) == "[error]")
-    {
-        retval << color::bold << color::red << "[error]" << color::reset
-               << color::bold << message.substr(7) << color::reset << '\n';
-    }
-    else
-    {
-        retval << color::bold << color::red << "[error] " << color::reset
-               << color::bold << message << color::reset << '\n';
-    }
-
-    const auto format_one_location = [line_num_width]
-        (std::ostringstream& oss,
-         const source_location& loc, const std::string& comment) -> void
-        {
-            oss << ' ' << color::bold << color::blue
-                << std::setw(static_cast<int>(line_num_width))
-                << std::right << loc.line() << " | "  << color::reset
-                << loc.line_str() << '\n';
-
-            oss << make_string(line_num_width + 1, ' ')
-                << color::bold << color::blue << " | " << color::reset
-                << make_string(loc.column()-1 /*1-origin*/, ' ');
-
-            if(loc.region() == 1)
-            {
-                // invalid
-                // ^------
-                oss << color::bold << color::red << "^---" << color::reset;
-            }
-            else
-            {
-                // invalid
-                // ~~~~~~~
-                const auto underline_len = (std::min)(
-                    static_cast<std::size_t>(loc.region()), loc.line_str().size());
-                oss << color::bold << color::red
-                    << make_string(underline_len, '~') << color::reset;
-            }
-            oss << ' ';
-            oss << comment;
-            return;
-        };
-
-    assert(!loc_com.empty());
-
-    // --> example.toml
-    //   |
-    retval << color::bold << color::blue << " --> " << color::reset
-           << loc_com.front().first.file_name() << '\n';
-    retval << make_string(line_num_width + 1, ' ')
-           << color::bold << color::blue << " |\n"  << color::reset;
-    // 1 | key value
-    //   |    ^--- missing =
-    format_one_location(retval, loc_com.front().first, loc_com.front().second);
-
-    // process the rest of the locations
-    for(std::size_t i=1; i<loc_com.size(); ++i)
-    {
-        const auto& prev = loc_com.at(i-1);
-        const auto& curr = loc_com.at(i);
-
-        retval << '\n';
-        // if the filenames are the same, print "..."
-        if(prev.first.file_name() == curr.first.file_name())
-        {
-            retval << color::bold << color::blue << " ...\n" << color::reset;
-        }
-        else // if filename differs, print " --> filename.toml" again
-        {
-            retval << color::bold << color::blue << " --> " << color::reset
-                   << curr.first.file_name() << '\n';
-            retval << make_string(line_num_width + 1, ' ')
-                   << color::bold << color::blue << " |\n"  << color::reset;
-        }
-
-        format_one_location(retval, curr.first, curr.second);
-    }
-
-    if(!helps.empty())
-    {
-        retval << '\n';
-        retval << make_string(line_num_width + 1, ' ');
-        retval << color::bold << color::blue << " |" << color::reset;
-        for(const auto& help : helps)
-        {
-            retval << color::bold << "\nHint: " << color::reset;
-            retval << help;
-        }
-    }
-    return retval.str();
-}
-
-} // detail
-} // toml
-#endif// TOML11_SOURCE_LOCATION_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/storage.hpp b/src/frontend/qt_sdl/toml/toml/storage.hpp
deleted file mode 100644
index 202f9035..00000000
--- a/src/frontend/qt_sdl/toml/toml/storage.hpp
+++ /dev/null
@@ -1,43 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_STORAGE_HPP
-#define TOML11_STORAGE_HPP
-#include "utility.hpp"
-
-namespace toml
-{
-namespace detail
-{
-
-// this contains pointer and deep-copy the content if copied.
-// to avoid recursive pointer.
-template<typename T>
-struct storage
-{
-    using value_type = T;
-
-    explicit storage(value_type const& v): ptr(toml::make_unique<T>(v)) {}
-    explicit storage(value_type&&      v): ptr(toml::make_unique<T>(std::move(v))) {}
-    ~storage() = default;
-    storage(const storage& rhs): ptr(toml::make_unique<T>(*rhs.ptr)) {}
-    storage& operator=(const storage& rhs)
-    {
-        this->ptr = toml::make_unique<T>(*rhs.ptr);
-        return *this;
-    }
-    storage(storage&&) = default;
-    storage& operator=(storage&&) = default;
-
-    bool is_ok() const noexcept {return static_cast<bool>(ptr);}
-
-    value_type&       value() &      noexcept {return *ptr;}
-    value_type const& value() const& noexcept {return *ptr;}
-    value_type&&      value() &&     noexcept {return std::move(*ptr);}
-
-  private:
-    std::unique_ptr<value_type> ptr;
-};
-
-} // detail
-} // toml
-#endif// TOML11_STORAGE_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/string.hpp b/src/frontend/qt_sdl/toml/toml/string.hpp
deleted file mode 100644
index def3e57c..00000000
--- a/src/frontend/qt_sdl/toml/toml/string.hpp
+++ /dev/null
@@ -1,228 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_STRING_HPP
-#define TOML11_STRING_HPP
-
-#include "version.hpp"
-
-#include <cstdint>
-
-#include <algorithm>
-#include <string>
-
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
-#if __has_include(<string_view>)
-#define TOML11_USING_STRING_VIEW 1
-#include <string_view>
-#endif
-#endif
-
-namespace toml
-{
-
-enum class string_t : std::uint8_t
-{
-    basic   = 0,
-    literal = 1,
-};
-
-struct string
-{
-    string()  = default;
-    ~string() = default;
-    string(const string& s) = default;
-    string(string&& s)      = default;
-    string& operator=(const string& s) = default;
-    string& operator=(string&& s)      = default;
-
-    string(const std::string& s): kind(string_t::basic), str(s){}
-    string(const std::string& s, string_t k):   kind(k), str(s){}
-    string(const char* s):        kind(string_t::basic), str(s){}
-    string(const char* s,        string_t k):   kind(k), str(s){}
-
-    string(std::string&& s): kind(string_t::basic), str(std::move(s)){}
-    string(std::string&& s, string_t k):   kind(k), str(std::move(s)){}
-
-    string& operator=(const std::string& s)
-    {kind = string_t::basic; str = s; return *this;}
-    string& operator=(std::string&& s)
-    {kind = string_t::basic; str = std::move(s); return *this;}
-
-    operator std::string&       () &      noexcept {return str;}
-    operator std::string const& () const& noexcept {return str;}
-    operator std::string&&      () &&     noexcept {return std::move(str);}
-
-    string& operator+=(const char*        rhs) {str += rhs; return *this;}
-    string& operator+=(const char         rhs) {str += rhs; return *this;}
-    string& operator+=(const std::string& rhs) {str += rhs; return *this;}
-    string& operator+=(const string&      rhs) {str += rhs.str; return *this;}
-
-#if defined(TOML11_USING_STRING_VIEW) && TOML11_USING_STRING_VIEW>0
-    explicit string(std::string_view s): kind(string_t::basic), str(s){}
-    string(std::string_view s, string_t k): kind(k), str(s){}
-
-    string& operator=(std::string_view s)
-    {kind = string_t::basic; str = s; return *this;}
-
-    explicit operator std::string_view() const noexcept
-    {return std::string_view(str);}
-
-    string& operator+=(const std::string_view& rhs) {str += rhs; return *this;}
-#endif
-
-    string_t    kind;
-    std::string str;
-};
-
-inline bool operator==(const string& lhs, const string& rhs)
-{
-    return lhs.kind == rhs.kind && lhs.str == rhs.str;
-}
-inline bool operator!=(const string& lhs, const string& rhs)
-{
-    return !(lhs == rhs);
-}
-inline bool operator<(const string& lhs, const string& rhs)
-{
-    return (lhs.kind == rhs.kind) ? (lhs.str < rhs.str) : (lhs.kind < rhs.kind);
-}
-inline bool operator>(const string& lhs, const string& rhs)
-{
-    return rhs < lhs;
-}
-inline bool operator<=(const string& lhs, const string& rhs)
-{
-    return !(rhs < lhs);
-}
-inline bool operator>=(const string& lhs, const string& rhs)
-{
-    return !(lhs < rhs);
-}
-
-inline bool
-operator==(const string& lhs, const std::string& rhs) {return lhs.str == rhs;}
-inline bool
-operator!=(const string& lhs, const std::string& rhs) {return lhs.str != rhs;}
-inline bool
-operator< (const string& lhs, const std::string& rhs) {return lhs.str <  rhs;}
-inline bool
-operator> (const string& lhs, const std::string& rhs) {return lhs.str >  rhs;}
-inline bool
-operator<=(const string& lhs, const std::string& rhs) {return lhs.str <= rhs;}
-inline bool
-operator>=(const string& lhs, const std::string& rhs) {return lhs.str >= rhs;}
-
-inline bool
-operator==(const std::string& lhs, const string& rhs) {return lhs == rhs.str;}
-inline bool
-operator!=(const std::string& lhs, const string& rhs) {return lhs != rhs.str;}
-inline bool
-operator< (const std::string& lhs, const string& rhs) {return lhs <  rhs.str;}
-inline bool
-operator> (const std::string& lhs, const string& rhs) {return lhs >  rhs.str;}
-inline bool
-operator<=(const std::string& lhs, const string& rhs) {return lhs <= rhs.str;}
-inline bool
-operator>=(const std::string& lhs, const string& rhs) {return lhs >= rhs.str;}
-
-inline bool
-operator==(const string& lhs, const char* rhs) {return lhs.str == std::string(rhs);}
-inline bool
-operator!=(const string& lhs, const char* rhs) {return lhs.str != std::string(rhs);}
-inline bool
-operator< (const string& lhs, const char* rhs) {return lhs.str <  std::string(rhs);}
-inline bool
-operator> (const string& lhs, const char* rhs) {return lhs.str >  std::string(rhs);}
-inline bool
-operator<=(const string& lhs, const char* rhs) {return lhs.str <= std::string(rhs);}
-inline bool
-operator>=(const string& lhs, const char* rhs) {return lhs.str >= std::string(rhs);}
-
-inline bool
-operator==(const char* lhs, const string& rhs) {return std::string(lhs) == rhs.str;}
-inline bool
-operator!=(const char* lhs, const string& rhs) {return std::string(lhs) != rhs.str;}
-inline bool
-operator< (const char* lhs, const string& rhs) {return std::string(lhs) <  rhs.str;}
-inline bool
-operator> (const char* lhs, const string& rhs) {return std::string(lhs) >  rhs.str;}
-inline bool
-operator<=(const char* lhs, const string& rhs) {return std::string(lhs) <= rhs.str;}
-inline bool
-operator>=(const char* lhs, const string& rhs) {return std::string(lhs) >= rhs.str;}
-
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, const string& s)
-{
-    if(s.kind == string_t::basic)
-    {
-        if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend())
-        {
-            // it contains newline. make it multiline string.
-            os << "\"\"\"\n";
-            for(auto i=s.str.cbegin(), e=s.str.cend(); i!=e; ++i)
-            {
-                switch(*i)
-                {
-                    case '\\': {os << "\\\\"; break;}
-                    case '\"': {os << "\\\""; break;}
-                    case '\b': {os << "\\b";  break;}
-                    case '\t': {os << "\\t";  break;}
-                    case '\f': {os << "\\f";  break;}
-                    case '\n': {os << '\n';   break;}
-                    case '\r':
-                    {
-                        // since it is a multiline string,
-                        // CRLF is not needed to be escaped.
-                        if(std::next(i) != e && *std::next(i) == '\n')
-                        {
-                            os << "\r\n";
-                            ++i;
-                        }
-                        else
-                        {
-                            os << "\\r";
-                        }
-                        break;
-                    }
-                    default: {os << *i; break;}
-                }
-            }
-            os << "\\\n\"\"\"";
-            return os;
-        }
-        // no newline. make it inline.
-        os << "\"";
-        for(const auto c : s.str)
-        {
-            switch(c)
-            {
-                case '\\': {os << "\\\\"; break;}
-                case '\"': {os << "\\\""; break;}
-                case '\b': {os << "\\b";  break;}
-                case '\t': {os << "\\t";  break;}
-                case '\f': {os << "\\f";  break;}
-                case '\n': {os << "\\n";  break;}
-                case '\r': {os << "\\r";  break;}
-                default  : {os << c;      break;}
-            }
-        }
-        os << "\"";
-        return os;
-    }
-    // the string `s` is literal-string.
-    if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend() ||
-       std::find(s.str.cbegin(), s.str.cend(), '\'') != s.str.cend() )
-    {
-        // contains newline or single quote. make it multiline.
-        os << "'''\n" << s.str << "'''";
-        return os;
-    }
-    // normal literal string
-    os << '\'' << s.str << '\'';
-    return os;
-}
-
-} // toml
-#endif// TOML11_STRING_H
diff --git a/src/frontend/qt_sdl/toml/toml/traits.hpp b/src/frontend/qt_sdl/toml/toml/traits.hpp
deleted file mode 100644
index 255d9e88..00000000
--- a/src/frontend/qt_sdl/toml/toml/traits.hpp
+++ /dev/null
@@ -1,328 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_TRAITS_HPP
-#define TOML11_TRAITS_HPP
-
-#include "from.hpp"
-#include "into.hpp"
-#include "version.hpp"
-
-#include <chrono>
-#include <forward_list>
-#include <string>
-#include <tuple>
-#include <type_traits>
-#include <utility>
-
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
-#if __has_include(<string_view>)
-#include <string_view>
-#endif // has_include(<string_view>)
-#endif // cplusplus   >= C++17
-
-namespace toml
-{
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-class basic_value;
-
-namespace detail
-{
-// ---------------------------------------------------------------------------
-// check whether type T is a kind of container/map class
-
-struct has_iterator_impl
-{
-    template<typename T> static std::true_type  check(typename T::iterator*);
-    template<typename T> static std::false_type check(...);
-};
-struct has_value_type_impl
-{
-    template<typename T> static std::true_type  check(typename T::value_type*);
-    template<typename T> static std::false_type check(...);
-};
-struct has_key_type_impl
-{
-    template<typename T> static std::true_type  check(typename T::key_type*);
-    template<typename T> static std::false_type check(...);
-};
-struct has_mapped_type_impl
-{
-    template<typename T> static std::true_type  check(typename T::mapped_type*);
-    template<typename T> static std::false_type check(...);
-};
-struct has_reserve_method_impl
-{
-    template<typename T> static std::false_type check(...);
-    template<typename T> static std::true_type  check(
-        decltype(std::declval<T>().reserve(std::declval<std::size_t>()))*);
-};
-struct has_push_back_method_impl
-{
-    template<typename T> static std::false_type check(...);
-    template<typename T> static std::true_type  check(
-        decltype(std::declval<T>().push_back(std::declval<typename T::value_type>()))*);
-};
-struct is_comparable_impl
-{
-    template<typename T> static std::false_type check(...);
-    template<typename T> static std::true_type  check(
-        decltype(std::declval<T>() < std::declval<T>())*);
-};
-
-struct has_from_toml_method_impl
-{
-    template<typename T, typename C,
-             template<typename ...> class Tb, template<typename ...> class A>
-    static std::true_type  check(
-        decltype(std::declval<T>().from_toml(
-                std::declval<::toml::basic_value<C, Tb, A>>()))*);
-
-    template<typename T, typename C,
-             template<typename ...> class Tb, template<typename ...> class A>
-    static std::false_type check(...);
-};
-struct has_into_toml_method_impl
-{
-    template<typename T>
-    static std::true_type  check(decltype(std::declval<T>().into_toml())*);
-    template<typename T>
-    static std::false_type check(...);
-};
-
-struct has_specialized_from_impl
-{
-    template<typename T>
-    static std::false_type check(...);
-    template<typename T, std::size_t S = sizeof(::toml::from<T>)>
-    static std::true_type check(::toml::from<T>*);
-};
-struct has_specialized_into_impl
-{
-    template<typename T>
-    static std::false_type check(...);
-    template<typename T, std::size_t S = sizeof(::toml::into<T>)>
-    static std::true_type check(::toml::from<T>*);
-};
-
-
-/// Intel C++ compiler can not use decltype in parent class declaration, here
-/// is a hack to work around it. https://stackoverflow.com/a/23953090/4692076
-#ifdef __INTEL_COMPILER
-#define decltype(...) std::enable_if<true, decltype(__VA_ARGS__)>::type
-#endif
-
-template<typename T>
-struct has_iterator    : decltype(has_iterator_impl::check<T>(nullptr)){};
-template<typename T>
-struct has_value_type  : decltype(has_value_type_impl::check<T>(nullptr)){};
-template<typename T>
-struct has_key_type    : decltype(has_key_type_impl::check<T>(nullptr)){};
-template<typename T>
-struct has_mapped_type : decltype(has_mapped_type_impl::check<T>(nullptr)){};
-template<typename T>
-struct has_reserve_method : decltype(has_reserve_method_impl::check<T>(nullptr)){};
-template<typename T>
-struct has_push_back_method : decltype(has_push_back_method_impl::check<T>(nullptr)){};
-template<typename T>
-struct is_comparable : decltype(is_comparable_impl::check<T>(nullptr)){};
-
-template<typename T, typename C,
-         template<typename ...> class Tb, template<typename ...> class A>
-struct has_from_toml_method
-: decltype(has_from_toml_method_impl::check<T, C, Tb, A>(nullptr)){};
-
-template<typename T>
-struct has_into_toml_method
-: decltype(has_into_toml_method_impl::check<T>(nullptr)){};
-
-template<typename T>
-struct has_specialized_from : decltype(has_specialized_from_impl::check<T>(nullptr)){};
-template<typename T>
-struct has_specialized_into : decltype(has_specialized_into_impl::check<T>(nullptr)){};
-
-#ifdef __INTEL_COMPILER
-#undef decltype
-#endif
-
-// ---------------------------------------------------------------------------
-// C++17 and/or/not
-
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
-
-using std::conjunction;
-using std::disjunction;
-using std::negation;
-
-#else
-
-template<typename ...> struct conjunction : std::true_type{};
-template<typename T>   struct conjunction<T> : T{};
-template<typename T, typename ... Ts>
-struct conjunction<T, Ts...> :
-    std::conditional<static_cast<bool>(T::value), conjunction<Ts...>, T>::type
-{};
-
-template<typename ...> struct disjunction : std::false_type{};
-template<typename T>   struct disjunction<T> : T {};
-template<typename T, typename ... Ts>
-struct disjunction<T, Ts...> :
-    std::conditional<static_cast<bool>(T::value), T, disjunction<Ts...>>::type
-{};
-
-template<typename T>
-struct negation : std::integral_constant<bool, !static_cast<bool>(T::value)>{};
-
-#endif
-
-// ---------------------------------------------------------------------------
-// type checkers
-
-template<typename T> struct is_std_pair : std::false_type{};
-template<typename T1, typename T2>
-struct is_std_pair<std::pair<T1, T2>> : std::true_type{};
-
-template<typename T> struct is_std_tuple : std::false_type{};
-template<typename ... Ts>
-struct is_std_tuple<std::tuple<Ts...>> : std::true_type{};
-
-template<typename T> struct is_std_forward_list : std::false_type{};
-template<typename T>
-struct is_std_forward_list<std::forward_list<T>> : std::true_type{};
-
-template<typename T> struct is_chrono_duration: std::false_type{};
-template<typename Rep, typename Period>
-struct is_chrono_duration<std::chrono::duration<Rep, Period>>: std::true_type{};
-
-template<typename T>
-struct is_map : conjunction< // map satisfies all the following conditions
-    has_iterator<T>,         // has T::iterator
-    has_value_type<T>,       // has T::value_type
-    has_key_type<T>,         // has T::key_type
-    has_mapped_type<T>       // has T::mapped_type
-    >{};
-template<typename T> struct is_map<T&>                : is_map<T>{};
-template<typename T> struct is_map<T const&>          : is_map<T>{};
-template<typename T> struct is_map<T volatile&>       : is_map<T>{};
-template<typename T> struct is_map<T const volatile&> : is_map<T>{};
-
-template<typename T>
-struct is_container : conjunction<
-    negation<is_map<T>>,                         // not a map
-    negation<std::is_same<T, std::string>>,      // not a std::string
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
-#if __has_include(<string_view>)
-    negation<std::is_same<T, std::string_view>>, // not a std::string_view
-#endif // has_include(<string_view>)
-#endif
-    has_iterator<T>,                             // has T::iterator
-    has_value_type<T>                            // has T::value_type
-    >{};
-template<typename T> struct is_container<T&>                : is_container<T>{};
-template<typename T> struct is_container<T const&>          : is_container<T>{};
-template<typename T> struct is_container<T volatile&>       : is_container<T>{};
-template<typename T> struct is_container<T const volatile&> : is_container<T>{};
-
-template<typename T>
-struct is_basic_value: std::false_type{};
-template<typename T> struct is_basic_value<T&>                : is_basic_value<T>{};
-template<typename T> struct is_basic_value<T const&>          : is_basic_value<T>{};
-template<typename T> struct is_basic_value<T volatile&>       : is_basic_value<T>{};
-template<typename T> struct is_basic_value<T const volatile&> : is_basic_value<T>{};
-template<typename C, template<typename ...> class M, template<typename ...> class V>
-struct is_basic_value<::toml::basic_value<C, M, V>>: std::true_type{};
-
-// ---------------------------------------------------------------------------
-// C++14 index_sequence
-
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
-
-using std::index_sequence;
-using std::make_index_sequence;
-
-#else
-
-template<std::size_t ... Ns> struct index_sequence{};
-
-template<typename IS, std::size_t N> struct push_back_index_sequence{};
-template<std::size_t N, std::size_t ... Ns>
-struct push_back_index_sequence<index_sequence<Ns...>, N>
-{
-    typedef index_sequence<Ns..., N> type;
-};
-
-template<std::size_t N>
-struct index_sequence_maker
-{
-    typedef typename push_back_index_sequence<
-        typename index_sequence_maker<N-1>::type, N>::type type;
-};
-template<>
-struct index_sequence_maker<0>
-{
-    typedef index_sequence<0> type;
-};
-template<std::size_t N>
-using make_index_sequence = typename index_sequence_maker<N-1>::type;
-
-#endif // cplusplus >= 2014
-
-// ---------------------------------------------------------------------------
-// C++14 enable_if_t
-
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
-
-using std::enable_if_t;
-
-#else
-
-template<bool B, typename T>
-using enable_if_t = typename std::enable_if<B, T>::type;
-
-#endif // cplusplus >= 2014
-
-// ---------------------------------------------------------------------------
-// return_type_of_t
-
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L && defined(__cpp_lib_is_invocable) && __cpp_lib_is_invocable>=201703
-
-template<typename F, typename ... Args>
-using return_type_of_t = std::invoke_result_t<F, Args...>;
-
-#else
-// result_of is deprecated after C++17
-template<typename F, typename ... Args>
-using return_type_of_t = typename std::result_of<F(Args...)>::type;
-
-#endif
-
-// ---------------------------------------------------------------------------
-// is_string_literal
-//
-// to use this, pass `typename remove_reference<T>::type` to T.
-
-template<typename T>
-struct is_string_literal:
-disjunction<
-    std::is_same<const char*, T>,
-    conjunction<
-        std::is_array<T>,
-        std::is_same<const char, typename std::remove_extent<T>::type>
-        >
-    >{};
-
-// ---------------------------------------------------------------------------
-// C++20 remove_cvref_t
-
-template<typename T>
-struct remove_cvref
-{
-    using type = typename std::remove_cv<
-        typename std::remove_reference<T>::type>::type;
-};
-
-template<typename T>
-using remove_cvref_t = typename remove_cvref<T>::type;
-
-}// detail
-}//toml
-#endif // TOML_TRAITS
diff --git a/src/frontend/qt_sdl/toml/toml/types.hpp b/src/frontend/qt_sdl/toml/toml/types.hpp
deleted file mode 100644
index 1e420e7f..00000000
--- a/src/frontend/qt_sdl/toml/toml/types.hpp
+++ /dev/null
@@ -1,173 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_TYPES_HPP
-#define TOML11_TYPES_HPP
-#include <unordered_map>
-#include <vector>
-
-#include "comments.hpp"
-#include "datetime.hpp"
-#include "string.hpp"
-#include "traits.hpp"
-
-namespace toml
-{
-
-template<typename Comment, // discard/preserve_comment
-         template<typename ...> class Table, // map-like class
-         template<typename ...> class Array> // vector-like class
-class basic_value;
-
-using character = char;
-using key = std::string;
-
-#if !defined(__clang__) && defined(__GNUC__) && __GNUC__ <= 4
-#  pragma GCC diagnostic push
-#  pragma GCC diagnostic ignored "-Wshadow"
-#endif
-
-using boolean        = bool;
-using integer        = std::int64_t;
-using floating       = double; // "float" is a keyword, cannot use it here.
-// the following stuffs are structs defined here, so aliases are not needed.
-// - string
-// - offset_datetime
-// - offset_datetime
-// - local_datetime
-// - local_date
-// - local_time
-
-#if defined(__GNUC__) && !defined(__clang__)
-#  pragma GCC diagnostic pop
-#endif
-
-// default toml::value and default array/table. these are defined after defining
-// basic_value itself.
-// using value = basic_value<discard_comments, std::unordered_map, std::vector>;
-// using array = typename value::array_type;
-// using table = typename value::table_type;
-
-// to avoid warnings about `value_t::integer` is "shadowing" toml::integer in
-// GCC -Wshadow=global.
-#if defined(__GNUC__) && !defined(__clang__)
-#  pragma GCC diagnostic push
-#  if 7 <= __GNUC__
-#    pragma GCC diagnostic ignored "-Wshadow=global"
-#  else // gcc-6 or older
-#    pragma GCC diagnostic ignored "-Wshadow"
-#  endif
-#endif
-enum class value_t : std::uint8_t
-{
-    empty           =  0,
-    boolean         =  1,
-    integer         =  2,
-    floating        =  3,
-    string          =  4,
-    offset_datetime =  5,
-    local_datetime  =  6,
-    local_date      =  7,
-    local_time      =  8,
-    array           =  9,
-    table           = 10,
-};
-#if defined(__GNUC__) && !defined(__clang__)
-#  pragma GCC diagnostic pop
-#endif
-
-template<typename charT, typename traits>
-inline std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, value_t t)
-{
-    switch(t)
-    {
-        case value_t::boolean         : os << "boolean";         return os;
-        case value_t::integer         : os << "integer";         return os;
-        case value_t::floating        : os << "floating";        return os;
-        case value_t::string          : os << "string";          return os;
-        case value_t::offset_datetime : os << "offset_datetime"; return os;
-        case value_t::local_datetime  : os << "local_datetime";  return os;
-        case value_t::local_date      : os << "local_date";      return os;
-        case value_t::local_time      : os << "local_time";      return os;
-        case value_t::array           : os << "array";           return os;
-        case value_t::table           : os << "table";           return os;
-        case value_t::empty           : os << "empty";           return os;
-        default                       : os << "unknown";         return os;
-    }
-}
-
-template<typename charT = char,
-         typename traits = std::char_traits<charT>,
-         typename alloc = std::allocator<charT>>
-inline std::basic_string<charT, traits, alloc> stringize(value_t t)
-{
-    std::basic_ostringstream<charT, traits, alloc> oss;
-    oss << t;
-    return oss.str();
-}
-
-namespace detail
-{
-
-// helper to define a type that represents a value_t value.
-template<value_t V>
-using value_t_constant = std::integral_constant<value_t, V>;
-
-// meta-function that convertes from value_t to the exact toml type that corresponds to.
-// It takes toml::basic_value type because array and table types depend on it.
-template<value_t t, typename Value> struct enum_to_type                      {using type = void                      ;};
-template<typename Value> struct enum_to_type<value_t::empty          , Value>{using type = void                      ;};
-template<typename Value> struct enum_to_type<value_t::boolean        , Value>{using type = boolean                   ;};
-template<typename Value> struct enum_to_type<value_t::integer        , Value>{using type = integer                   ;};
-template<typename Value> struct enum_to_type<value_t::floating       , Value>{using type = floating                  ;};
-template<typename Value> struct enum_to_type<value_t::string         , Value>{using type = string                    ;};
-template<typename Value> struct enum_to_type<value_t::offset_datetime, Value>{using type = offset_datetime           ;};
-template<typename Value> struct enum_to_type<value_t::local_datetime , Value>{using type = local_datetime            ;};
-template<typename Value> struct enum_to_type<value_t::local_date     , Value>{using type = local_date                ;};
-template<typename Value> struct enum_to_type<value_t::local_time     , Value>{using type = local_time                ;};
-template<typename Value> struct enum_to_type<value_t::array          , Value>{using type = typename Value::array_type;};
-template<typename Value> struct enum_to_type<value_t::table          , Value>{using type = typename Value::table_type;};
-
-// meta-function that converts from an exact toml type to the enum that corresponds to.
-template<typename T, typename Value>
-struct type_to_enum : std::conditional<
-    std::is_same<T, typename Value::array_type>::value, // if T == array_type,
-    value_t_constant<value_t::array>,                   // then value_t::array
-    typename std::conditional<                          // else...
-        std::is_same<T, typename Value::table_type>::value, // if T == table_type
-        value_t_constant<value_t::table>,               // then value_t::table
-        value_t_constant<value_t::empty>                // else value_t::empty
-        >::type
-    >::type {};
-template<typename Value> struct type_to_enum<boolean        , Value>: value_t_constant<value_t::boolean        > {};
-template<typename Value> struct type_to_enum<integer        , Value>: value_t_constant<value_t::integer        > {};
-template<typename Value> struct type_to_enum<floating       , Value>: value_t_constant<value_t::floating       > {};
-template<typename Value> struct type_to_enum<string         , Value>: value_t_constant<value_t::string         > {};
-template<typename Value> struct type_to_enum<offset_datetime, Value>: value_t_constant<value_t::offset_datetime> {};
-template<typename Value> struct type_to_enum<local_datetime , Value>: value_t_constant<value_t::local_datetime > {};
-template<typename Value> struct type_to_enum<local_date     , Value>: value_t_constant<value_t::local_date     > {};
-template<typename Value> struct type_to_enum<local_time     , Value>: value_t_constant<value_t::local_time     > {};
-
-// meta-function that checks the type T is the same as one of the toml::* types.
-template<typename T, typename Value>
-struct is_exact_toml_type : disjunction<
-    std::is_same<T, boolean        >,
-    std::is_same<T, integer        >,
-    std::is_same<T, floating       >,
-    std::is_same<T, string         >,
-    std::is_same<T, offset_datetime>,
-    std::is_same<T, local_datetime >,
-    std::is_same<T, local_date     >,
-    std::is_same<T, local_time     >,
-    std::is_same<T, typename Value::array_type>,
-    std::is_same<T, typename Value::table_type>
-    >{};
-template<typename T, typename V> struct is_exact_toml_type<T&, V>               : is_exact_toml_type<T, V>{};
-template<typename T, typename V> struct is_exact_toml_type<T const&, V>         : is_exact_toml_type<T, V>{};
-template<typename T, typename V> struct is_exact_toml_type<T volatile&, V>      : is_exact_toml_type<T, V>{};
-template<typename T, typename V> struct is_exact_toml_type<T const volatile&, V>: is_exact_toml_type<T, V>{};
-
-} // detail
-} // toml
-
-#endif// TOML11_TYPES_H
diff --git a/src/frontend/qt_sdl/toml/toml/utility.hpp b/src/frontend/qt_sdl/toml/toml/utility.hpp
deleted file mode 100644
index 53a18b94..00000000
--- a/src/frontend/qt_sdl/toml/toml/utility.hpp
+++ /dev/null
@@ -1,150 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_UTILITY_HPP
-#define TOML11_UTILITY_HPP
-#include <memory>
-#include <sstream>
-#include <utility>
-
-#include "traits.hpp"
-#include "version.hpp"
-
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
-#  define TOML11_MARK_AS_DEPRECATED(msg) [[deprecated(msg)]]
-#elif defined(__GNUC__)
-#  define TOML11_MARK_AS_DEPRECATED(msg) __attribute__((deprecated(msg)))
-#elif defined(_MSC_VER)
-#  define TOML11_MARK_AS_DEPRECATED(msg) __declspec(deprecated(msg))
-#else
-#  define TOML11_MARK_AS_DEPRECATED
-#endif
-
-namespace toml
-{
-
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
-
-using std::make_unique;
-
-#else
-
-template<typename T, typename ... Ts>
-inline std::unique_ptr<T> make_unique(Ts&& ... args)
-{
-    return std::unique_ptr<T>(new T(std::forward<Ts>(args)...));
-}
-
-#endif // TOML11_CPLUSPLUS_STANDARD_VERSION >= 2014
-
-namespace detail
-{
-template<typename Container>
-void try_reserve_impl(Container& container, std::size_t N, std::true_type)
-{
-    container.reserve(N);
-    return;
-}
-template<typename Container>
-void try_reserve_impl(Container&, std::size_t, std::false_type) noexcept
-{
-    return;
-}
-} // detail
-
-template<typename Container>
-void try_reserve(Container& container, std::size_t N)
-{
-    if(N <= container.size()) {return;}
-    detail::try_reserve_impl(container, N, detail::has_reserve_method<Container>{});
-    return;
-}
-
-namespace detail
-{
-inline std::string concat_to_string_impl(std::ostringstream& oss)
-{
-    return oss.str();
-}
-template<typename T, typename ... Ts>
-std::string concat_to_string_impl(std::ostringstream& oss, T&& head, Ts&& ... tail)
-{
-    oss << std::forward<T>(head);
-    return concat_to_string_impl(oss, std::forward<Ts>(tail) ... );
-}
-} // detail
-
-template<typename ... Ts>
-std::string concat_to_string(Ts&& ... args)
-{
-    std::ostringstream oss;
-    oss << std::boolalpha << std::fixed;
-    return detail::concat_to_string_impl(oss, std::forward<Ts>(args) ...);
-}
-
-template<typename T>
-T from_string(const std::string& str, T opt)
-{
-    T v(opt);
-    std::istringstream iss(str);
-    iss >> v;
-    return v;
-}
-
-namespace detail
-{
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201402L
-template<typename T>
-decltype(auto) last_one(T&& tail) noexcept
-{
-    return std::forward<T>(tail);
-}
-
-template<typename T, typename ... Ts>
-decltype(auto) last_one(T&& /*head*/, Ts&& ... tail) noexcept
-{
-    return last_one(std::forward<Ts>(tail)...);
-}
-#else // C++11
-// The following code
-// ```cpp
-//  1 | template<typename T, typename ... Ts>
-//  2 | auto last_one(T&& /*head*/, Ts&& ... tail)
-//  3 |  -> decltype(last_one(std::forward<Ts>(tail)...))
-//  4 | {
-//  5 |     return last_one(std::forward<Ts>(tail)...);
-//  6 | }
-// ```
-// does not work because the function `last_one(...)` is not yet defined at
-// line #3, so `decltype()` cannot deduce the type returned from `last_one`.
-// So we need to determine return type in a different way, like a meta func.
-
-template<typename T, typename ... Ts>
-struct last_one_in_pack
-{
-    using type = typename last_one_in_pack<Ts...>::type;
-};
-template<typename T>
-struct last_one_in_pack<T>
-{
-    using type = T;
-};
-template<typename ... Ts>
-using last_one_in_pack_t = typename last_one_in_pack<Ts...>::type;
-
-template<typename T>
-T&& last_one(T&& tail) noexcept
-{
-    return std::forward<T>(tail);
-}
-template<typename T, typename ... Ts>
-enable_if_t<(sizeof...(Ts) > 0), last_one_in_pack_t<Ts&& ...>>
-last_one(T&& /*head*/, Ts&& ... tail)
-{
-    return last_one(std::forward<Ts>(tail)...);
-}
-
-#endif
-} // detail
-
-}// toml
-#endif // TOML11_UTILITY
diff --git a/src/frontend/qt_sdl/toml/toml/value.hpp b/src/frontend/qt_sdl/toml/toml/value.hpp
deleted file mode 100644
index 1b43db8d..00000000
--- a/src/frontend/qt_sdl/toml/toml/value.hpp
+++ /dev/null
@@ -1,2035 +0,0 @@
-//     Copyright Toru Niina 2017.
-// Distributed under the MIT License.
-#ifndef TOML11_VALUE_HPP
-#define TOML11_VALUE_HPP
-#include <cassert>
-
-#include "comments.hpp"
-#include "exception.hpp"
-#include "into.hpp"
-#include "region.hpp"
-#include "source_location.hpp"
-#include "storage.hpp"
-#include "traits.hpp"
-#include "types.hpp"
-#include "utility.hpp"
-
-namespace toml
-{
-
-namespace detail
-{
-
-// to show error messages. not recommended for users.
-template<typename Value>
-inline region_base const* get_region(const Value& v)
-{
-    return v.region_info_.get();
-}
-
-template<typename Value>
-void change_region(Value& v, region reg)
-{
-    v.region_info_ = std::make_shared<region>(std::move(reg));
-    return;
-}
-
-template<value_t Expected, typename Value>
-[[noreturn]] inline void
-throw_bad_cast(const std::string& funcname, value_t actual, const Value& v)
-{
-    throw type_error(detail::format_underline(
-        concat_to_string(funcname, "bad_cast to ", Expected), {
-            {v.location(), concat_to_string("the actual type is ", actual)}
-        }), v.location());
-}
-
-// Throw `out_of_range` from `toml::value::at()` and `toml::find()`
-// after generating an error message.
-//
-// The implementation is a bit complicated and there are many edge-cases.
-// If you are not interested in the error message generation, just skip this.
-template<typename Value>
-[[noreturn]] void
-throw_key_not_found_error(const Value& v, const key& ky)
-{
-    // The top-level table has its region at the first character of the file.
-    // That means that, in the case when a key is not found in the top-level
-    // table, the error message points to the first character. If the file has
-    // its first table at the first line, the error message would be like this.
-    // ```console
-    // [error] key "a" not found
-    //  --> example.toml
-    //    |
-    //  1 | [table]
-    //    | ^------ in this table
-    // ```
-    // It actually points to the top-level table at the first character,
-    // not `[table]`. But it is too confusing. To avoid the confusion, the error
-    // message should explicitly say "key not found in the top-level table",
-    // or "the parsed file is empty" if there is no content at all (0 bytes in file).
-    const auto loc = v.location();
-    if(loc.line() == 1 && loc.region() == 0)
-    {
-        // First line with a zero-length region means "empty file".
-        // The region will be generated at `parse_toml_file` function
-        // if the file contains no bytes.
-        throw std::out_of_range(format_underline(concat_to_string(
-            "key \"", ky, "\" not found in the top-level table"), {
-                {loc, "the parsed file is empty"}
-            }));
-    }
-    else if(loc.line() == 1 && loc.region() == 1)
-    {
-        // Here it assumes that top-level table starts at the first character.
-        // The region corresponds to the top-level table will be generated at
-        // `parse_toml_file` function.
-        //     It also assumes that the top-level table size is just one and
-        // the line number is `1`. It is always satisfied. And those conditions
-        // are satisfied only if the table is the top-level table.
-        //
-        // 1. one-character dot-key at the first line
-        // ```toml
-        // a.b = "c"
-        // ```
-        // toml11 counts whole key as the table key. Here, `a.b` is the region
-        // of the table "a". It could be counter intuitive, but it works.
-        // The size of the region is 3, not 1. The above example is the shortest
-        // dot-key example. The size cannot be 1.
-        //
-        // 2. one-character inline-table at the first line
-        // ```toml
-        // a = {b = "c"}
-        // ```
-        // toml11 considers the inline table body as the table region. Here,
-        // `{b = "c"}` is the region of the table "a". The size of the region
-        // is 9, not 1. The shotest inline table still has two characters, `{`
-        // and `}`. The size cannot be 1.
-        //
-        // 3. one-character table declaration at the first line
-        // ```toml
-        // [a]
-        // ```
-        // toml11 considers the whole table key as the table region. Here,
-        // `[a]` is the table region. The size is 3, not 1.
-        //
-        throw std::out_of_range(format_underline(concat_to_string(
-            "key \"", ky, "\" not found in the top-level table"), {
-                {loc, "the top-level table starts here"}
-            }));
-    }
-    else
-    {
-        // normal table.
-        throw std::out_of_range(format_underline(concat_to_string(
-            "key \"", ky, "\" not found"), { {loc, "in this table"} }));
-    }
-}
-
-// switch by `value_t` at the compile time.
-template<value_t T>
-struct switch_cast {};
-#define TOML11_GENERATE_SWITCH_CASTER(TYPE) \
-    template<>                                                           \
-    struct switch_cast<value_t::TYPE>                                    \
-    {                                                                    \
-        template<typename Value>                                         \
-        static typename Value::TYPE##_type& invoke(Value& v)             \
-        {                                                                \
-            return v.as_##TYPE();                                        \
-        }                                                                \
-        template<typename Value>                                         \
-        static typename Value::TYPE##_type const& invoke(const Value& v) \
-        {                                                                \
-            return v.as_##TYPE();                                        \
-        }                                                                \
-        template<typename Value>                                         \
-        static typename Value::TYPE##_type&& invoke(Value&& v)           \
-        {                                                                \
-            return std::move(v).as_##TYPE();                             \
-        }                                                                \
-    };                                                                   \
-    /**/
-TOML11_GENERATE_SWITCH_CASTER(boolean)
-TOML11_GENERATE_SWITCH_CASTER(integer)
-TOML11_GENERATE_SWITCH_CASTER(floating)
-TOML11_GENERATE_SWITCH_CASTER(string)
-TOML11_GENERATE_SWITCH_CASTER(offset_datetime)
-TOML11_GENERATE_SWITCH_CASTER(local_datetime)
-TOML11_GENERATE_SWITCH_CASTER(local_date)
-TOML11_GENERATE_SWITCH_CASTER(local_time)
-TOML11_GENERATE_SWITCH_CASTER(array)
-TOML11_GENERATE_SWITCH_CASTER(table)
-
-#undef TOML11_GENERATE_SWITCH_CASTER
-
-}// detail
-
-template<typename Comment, // discard/preserve_comment
-         template<typename ...> class Table = std::unordered_map,
-         template<typename ...> class Array = std::vector>
-class basic_value
-{
-    template<typename T, typename U>
-    static void assigner(T& dst, U&& v)
-    {
-        const auto tmp = ::new(std::addressof(dst)) T(std::forward<U>(v));
-        assert(tmp == std::addressof(dst));
-        (void)tmp;
-    }
-
-    using region_base = detail::region_base;
-
-    template<typename C, template<typename ...> class T,
-             template<typename ...> class A>
-    friend class basic_value;
-
-  public:
-
-    using comment_type         = Comment;
-    using key_type             = ::toml::key;
-    using value_type           = basic_value<comment_type, Table, Array>;
-    using boolean_type         = ::toml::boolean;
-    using integer_type         = ::toml::integer;
-    using floating_type        = ::toml::floating;
-    using string_type          = ::toml::string;
-    using local_time_type      = ::toml::local_time;
-    using local_date_type      = ::toml::local_date;
-    using local_datetime_type  = ::toml::local_datetime;
-    using offset_datetime_type = ::toml::offset_datetime;
-    using array_type           = Array<value_type>;
-    using table_type           = Table<key_type, value_type>;
-
-  public:
-
-    basic_value() noexcept
-        : type_(value_t::empty),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {}
-    ~basic_value() noexcept {this->cleanup();}
-
-    basic_value(const basic_value& v)
-        : type_(v.type()), region_info_(v.region_info_), comments_(v.comments_)
-    {
-        switch(v.type())
-        {
-            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
-            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
-            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
-            case value_t::string         : assigner(string_         , v.string_         ); break;
-            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
-            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
-            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
-            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
-            case value_t::array          : assigner(array_          , v.array_          ); break;
-            case value_t::table          : assigner(table_          , v.table_          ); break;
-            default: break;
-        }
-    }
-    basic_value(basic_value&& v)
-        : type_(v.type()), region_info_(std::move(v.region_info_)),
-          comments_(std::move(v.comments_))
-    {
-        switch(this->type_) // here this->type_ is already initialized
-        {
-            case value_t::boolean        : assigner(boolean_        , std::move(v.boolean_        )); break;
-            case value_t::integer        : assigner(integer_        , std::move(v.integer_        )); break;
-            case value_t::floating       : assigner(floating_       , std::move(v.floating_       )); break;
-            case value_t::string         : assigner(string_         , std::move(v.string_         )); break;
-            case value_t::offset_datetime: assigner(offset_datetime_, std::move(v.offset_datetime_)); break;
-            case value_t::local_datetime : assigner(local_datetime_ , std::move(v.local_datetime_ )); break;
-            case value_t::local_date     : assigner(local_date_     , std::move(v.local_date_     )); break;
-            case value_t::local_time     : assigner(local_time_     , std::move(v.local_time_     )); break;
-            case value_t::array          : assigner(array_          , std::move(v.array_          )); break;
-            case value_t::table          : assigner(table_          , std::move(v.table_          )); break;
-            default: break;
-        }
-    }
-    basic_value& operator=(const basic_value& v)
-    {
-        this->cleanup();
-        this->region_info_ = v.region_info_;
-        this->comments_ = v.comments_;
-        this->type_ = v.type();
-        switch(this->type_)
-        {
-            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
-            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
-            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
-            case value_t::string         : assigner(string_         , v.string_         ); break;
-            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
-            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
-            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
-            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
-            case value_t::array          : assigner(array_          , v.array_          ); break;
-            case value_t::table          : assigner(table_          , v.table_          ); break;
-            default: break;
-        }
-        return *this;
-    }
-    basic_value& operator=(basic_value&& v)
-    {
-        this->cleanup();
-        this->region_info_ = std::move(v.region_info_);
-        this->comments_ = std::move(v.comments_);
-        this->type_ = v.type();
-        switch(this->type_)
-        {
-            case value_t::boolean        : assigner(boolean_        , std::move(v.boolean_        )); break;
-            case value_t::integer        : assigner(integer_        , std::move(v.integer_        )); break;
-            case value_t::floating       : assigner(floating_       , std::move(v.floating_       )); break;
-            case value_t::string         : assigner(string_         , std::move(v.string_         )); break;
-            case value_t::offset_datetime: assigner(offset_datetime_, std::move(v.offset_datetime_)); break;
-            case value_t::local_datetime : assigner(local_datetime_ , std::move(v.local_datetime_ )); break;
-            case value_t::local_date     : assigner(local_date_     , std::move(v.local_date_     )); break;
-            case value_t::local_time     : assigner(local_time_     , std::move(v.local_time_     )); break;
-            case value_t::array          : assigner(array_          , std::move(v.array_          )); break;
-            case value_t::table          : assigner(table_          , std::move(v.table_          )); break;
-            default: break;
-        }
-        return *this;
-    }
-
-    // overwrite comments ----------------------------------------------------
-
-    basic_value(const basic_value& v, std::vector<std::string> com)
-        : type_(v.type()), region_info_(v.region_info_),
-          comments_(std::move(com))
-    {
-        switch(v.type())
-        {
-            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
-            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
-            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
-            case value_t::string         : assigner(string_         , v.string_         ); break;
-            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
-            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
-            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
-            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
-            case value_t::array          : assigner(array_          , v.array_          ); break;
-            case value_t::table          : assigner(table_          , v.table_          ); break;
-            default: break;
-        }
-    }
-
-    basic_value(basic_value&& v, std::vector<std::string> com)
-        : type_(v.type()), region_info_(std::move(v.region_info_)),
-          comments_(std::move(com))
-    {
-        switch(this->type_) // here this->type_ is already initialized
-        {
-            case value_t::boolean        : assigner(boolean_        , std::move(v.boolean_        )); break;
-            case value_t::integer        : assigner(integer_        , std::move(v.integer_        )); break;
-            case value_t::floating       : assigner(floating_       , std::move(v.floating_       )); break;
-            case value_t::string         : assigner(string_         , std::move(v.string_         )); break;
-            case value_t::offset_datetime: assigner(offset_datetime_, std::move(v.offset_datetime_)); break;
-            case value_t::local_datetime : assigner(local_datetime_ , std::move(v.local_datetime_ )); break;
-            case value_t::local_date     : assigner(local_date_     , std::move(v.local_date_     )); break;
-            case value_t::local_time     : assigner(local_time_     , std::move(v.local_time_     )); break;
-            case value_t::array          : assigner(array_          , std::move(v.array_          )); break;
-            case value_t::table          : assigner(table_          , std::move(v.table_          )); break;
-            default: break;
-        }
-    }
-
-    // -----------------------------------------------------------------------
-    // conversion between different basic_values.
-    template<typename C,
-             template<typename ...> class T,
-             template<typename ...> class A>
-    basic_value(const basic_value<C, T, A>& v)
-        : type_(v.type()), region_info_(v.region_info_), comments_(v.comments())
-    {
-        switch(v.type())
-        {
-            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
-            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
-            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
-            case value_t::string         : assigner(string_         , v.string_         ); break;
-            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
-            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
-            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
-            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
-            case value_t::array          :
-            {
-                array_type tmp(v.as_array(std::nothrow).begin(),
-                               v.as_array(std::nothrow).end());
-                assigner(array_, std::move(tmp));
-                break;
-            }
-            case value_t::table          :
-            {
-                table_type tmp(v.as_table(std::nothrow).begin(),
-                               v.as_table(std::nothrow).end());
-                assigner(table_, std::move(tmp));
-                break;
-            }
-            default: break;
-        }
-    }
-    template<typename C,
-             template<typename ...> class T,
-             template<typename ...> class A>
-    basic_value(const basic_value<C, T, A>& v, std::vector<std::string> com)
-        : type_(v.type()), region_info_(v.region_info_),
-          comments_(std::move(com))
-    {
-        switch(v.type())
-        {
-            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
-            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
-            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
-            case value_t::string         : assigner(string_         , v.string_         ); break;
-            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
-            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
-            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
-            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
-            case value_t::array          :
-            {
-                array_type tmp(v.as_array(std::nothrow).begin(),
-                               v.as_array(std::nothrow).end());
-                assigner(array_, std::move(tmp));
-                break;
-            }
-            case value_t::table          :
-            {
-                table_type tmp(v.as_table(std::nothrow).begin(),
-                               v.as_table(std::nothrow).end());
-                assigner(table_, std::move(tmp));
-                break;
-            }
-            default: break;
-        }
-    }
-    template<typename C,
-             template<typename ...> class T,
-             template<typename ...> class A>
-    basic_value& operator=(const basic_value<C, T, A>& v)
-    {
-        this->region_info_ = v.region_info_;
-        this->comments_    = comment_type(v.comments());
-        this->type_        = v.type();
-        switch(v.type())
-        {
-            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
-            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
-            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
-            case value_t::string         : assigner(string_         , v.string_         ); break;
-            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
-            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
-            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
-            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
-            case value_t::array          :
-            {
-                array_type tmp(v.as_array(std::nothrow).begin(),
-                               v.as_array(std::nothrow).end());
-                assigner(array_, std::move(tmp));
-                break;
-            }
-            case value_t::table          :
-            {
-                table_type tmp(v.as_table(std::nothrow).begin(),
-                               v.as_table(std::nothrow).end());
-                assigner(table_, std::move(tmp));
-                break;
-            }
-            default: break;
-        }
-        return *this;
-    }
-
-    // boolean ==============================================================
-
-    basic_value(boolean b)
-        : type_(value_t::boolean),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->boolean_, b);
-    }
-    basic_value& operator=(boolean b)
-    {
-        this->cleanup();
-        this->type_ = value_t::boolean;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->boolean_, b);
-        return *this;
-    }
-    basic_value(boolean b, std::vector<std::string> com)
-        : type_(value_t::boolean),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->boolean_, b);
-    }
-
-    // integer ==============================================================
-
-    template<typename T, typename std::enable_if<detail::conjunction<
-        std::is_integral<T>, detail::negation<std::is_same<T, boolean>>>::value,
-        std::nullptr_t>::type = nullptr>
-    basic_value(T i)
-        : type_(value_t::integer),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->integer_, static_cast<integer>(i));
-    }
-
-    template<typename T, typename std::enable_if<detail::conjunction<
-        std::is_integral<T>, detail::negation<std::is_same<T, boolean>>>::value,
-        std::nullptr_t>::type = nullptr>
-    basic_value& operator=(T i)
-    {
-        this->cleanup();
-        this->type_ = value_t::integer;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->integer_, static_cast<integer>(i));
-        return *this;
-    }
-
-    template<typename T, typename std::enable_if<detail::conjunction<
-        std::is_integral<T>, detail::negation<std::is_same<T, boolean>>>::value,
-        std::nullptr_t>::type = nullptr>
-    basic_value(T i, std::vector<std::string> com)
-        : type_(value_t::integer),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->integer_, static_cast<integer>(i));
-    }
-
-    // floating =============================================================
-
-    template<typename T, typename std::enable_if<
-        std::is_floating_point<T>::value, std::nullptr_t>::type = nullptr>
-    basic_value(T f)
-        : type_(value_t::floating),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->floating_, static_cast<floating>(f));
-    }
-
-
-    template<typename T, typename std::enable_if<
-        std::is_floating_point<T>::value, std::nullptr_t>::type = nullptr>
-    basic_value& operator=(T f)
-    {
-        this->cleanup();
-        this->type_ = value_t::floating;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->floating_, static_cast<floating>(f));
-        return *this;
-    }
-
-    template<typename T, typename std::enable_if<
-        std::is_floating_point<T>::value, std::nullptr_t>::type = nullptr>
-    basic_value(T f, std::vector<std::string> com)
-        : type_(value_t::floating),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->floating_, f);
-    }
-
-    // string ===============================================================
-
-    basic_value(toml::string s)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->string_, std::move(s));
-    }
-    basic_value& operator=(toml::string s)
-    {
-        this->cleanup();
-        this->type_ = value_t::string ;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->string_, s);
-        return *this;
-    }
-    basic_value(toml::string s, std::vector<std::string> com)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->string_, std::move(s));
-    }
-
-    basic_value(std::string s)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->string_, toml::string(std::move(s)));
-    }
-    basic_value& operator=(std::string s)
-    {
-        this->cleanup();
-        this->type_ = value_t::string ;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->string_, toml::string(std::move(s)));
-        return *this;
-    }
-    basic_value(std::string s, string_t kind)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->string_, toml::string(std::move(s), kind));
-    }
-    basic_value(std::string s, std::vector<std::string> com)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->string_, toml::string(std::move(s)));
-    }
-    basic_value(std::string s, string_t kind, std::vector<std::string> com)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->string_, toml::string(std::move(s), kind));
-    }
-
-    basic_value(const char* s)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->string_, toml::string(std::string(s)));
-    }
-    basic_value& operator=(const char* s)
-    {
-        this->cleanup();
-        this->type_ = value_t::string ;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->string_, toml::string(std::string(s)));
-        return *this;
-    }
-    basic_value(const char* s, string_t kind)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->string_, toml::string(std::string(s), kind));
-    }
-    basic_value(const char* s, std::vector<std::string> com)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->string_, toml::string(std::string(s)));
-    }
-    basic_value(const char* s, string_t kind, std::vector<std::string> com)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->string_, toml::string(std::string(s), kind));
-    }
-
-#if defined(TOML11_USING_STRING_VIEW) && TOML11_USING_STRING_VIEW>0
-    basic_value(std::string_view s)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->string_, toml::string(s));
-    }
-    basic_value& operator=(std::string_view s)
-    {
-        this->cleanup();
-        this->type_ = value_t::string ;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->string_, toml::string(s));
-        return *this;
-    }
-    basic_value(std::string_view s, std::vector<std::string> com)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->string_, toml::string(s));
-    }
-    basic_value(std::string_view s, string_t kind)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->string_, toml::string(s, kind));
-    }
-    basic_value(std::string_view s, string_t kind, std::vector<std::string> com)
-        : type_(value_t::string),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->string_, toml::string(s, kind));
-    }
-#endif
-
-    // local date ===========================================================
-
-    basic_value(const local_date& ld)
-        : type_(value_t::local_date),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->local_date_, ld);
-    }
-    basic_value& operator=(const local_date& ld)
-    {
-        this->cleanup();
-        this->type_ = value_t::local_date;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->local_date_, ld);
-        return *this;
-    }
-    basic_value(const local_date& ld, std::vector<std::string> com)
-        : type_(value_t::local_date),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->local_date_, ld);
-    }
-
-    // local time ===========================================================
-
-    basic_value(const local_time& lt)
-        : type_(value_t::local_time),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->local_time_, lt);
-    }
-    basic_value(const local_time& lt, std::vector<std::string> com)
-        : type_(value_t::local_time),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->local_time_, lt);
-    }
-    basic_value& operator=(const local_time& lt)
-    {
-        this->cleanup();
-        this->type_ = value_t::local_time;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->local_time_, lt);
-        return *this;
-    }
-
-    template<typename Rep, typename Period>
-    basic_value(const std::chrono::duration<Rep, Period>& dur)
-        : type_(value_t::local_time),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->local_time_, local_time(dur));
-    }
-    template<typename Rep, typename Period>
-    basic_value(const std::chrono::duration<Rep, Period>& dur,
-                std::vector<std::string> com)
-        : type_(value_t::local_time),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->local_time_, local_time(dur));
-    }
-    template<typename Rep, typename Period>
-    basic_value& operator=(const std::chrono::duration<Rep, Period>& dur)
-    {
-        this->cleanup();
-        this->type_ = value_t::local_time;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->local_time_, local_time(dur));
-        return *this;
-    }
-
-    // local datetime =======================================================
-
-    basic_value(const local_datetime& ldt)
-        : type_(value_t::local_datetime),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->local_datetime_, ldt);
-    }
-    basic_value(const local_datetime& ldt, std::vector<std::string> com)
-        : type_(value_t::local_datetime),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->local_datetime_, ldt);
-    }
-    basic_value& operator=(const local_datetime& ldt)
-    {
-        this->cleanup();
-        this->type_ = value_t::local_datetime;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->local_datetime_, ldt);
-        return *this;
-    }
-
-    // offset datetime ======================================================
-
-    basic_value(const offset_datetime& odt)
-        : type_(value_t::offset_datetime),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->offset_datetime_, odt);
-    }
-    basic_value(const offset_datetime& odt, std::vector<std::string> com)
-        : type_(value_t::offset_datetime),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->offset_datetime_, odt);
-    }
-    basic_value& operator=(const offset_datetime& odt)
-    {
-        this->cleanup();
-        this->type_ = value_t::offset_datetime;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->offset_datetime_, odt);
-        return *this;
-    }
-    basic_value(const std::chrono::system_clock::time_point& tp)
-        : type_(value_t::offset_datetime),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->offset_datetime_, offset_datetime(tp));
-    }
-    basic_value(const std::chrono::system_clock::time_point& tp,
-                std::vector<std::string> com)
-        : type_(value_t::offset_datetime),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->offset_datetime_, offset_datetime(tp));
-    }
-    basic_value& operator=(const std::chrono::system_clock::time_point& tp)
-    {
-        this->cleanup();
-        this->type_ = value_t::offset_datetime;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->offset_datetime_, offset_datetime(tp));
-        return *this;
-    }
-
-    // array ================================================================
-
-    basic_value(const array_type& ary)
-        : type_(value_t::array),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->array_, ary);
-    }
-    basic_value(const array_type& ary, std::vector<std::string> com)
-        : type_(value_t::array),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->array_, ary);
-    }
-    basic_value& operator=(const array_type& ary)
-    {
-        this->cleanup();
-        this->type_ = value_t::array ;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->array_, ary);
-        return *this;
-    }
-
-    // array (initializer_list) ----------------------------------------------
-
-    template<typename T, typename std::enable_if<
-            std::is_convertible<T, value_type>::value,
-        std::nullptr_t>::type = nullptr>
-    basic_value(std::initializer_list<T> list)
-        : type_(value_t::array),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        array_type ary(list.begin(), list.end());
-        assigner(this->array_, std::move(ary));
-    }
-    template<typename T, typename std::enable_if<
-            std::is_convertible<T, value_type>::value,
-        std::nullptr_t>::type = nullptr>
-    basic_value(std::initializer_list<T> list, std::vector<std::string> com)
-        : type_(value_t::array),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        array_type ary(list.begin(), list.end());
-        assigner(this->array_, std::move(ary));
-    }
-    template<typename T, typename std::enable_if<
-            std::is_convertible<T, value_type>::value,
-        std::nullptr_t>::type = nullptr>
-    basic_value& operator=(std::initializer_list<T> list)
-    {
-        this->cleanup();
-        this->type_ = value_t::array;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-
-        array_type ary(list.begin(), list.end());
-        assigner(this->array_, std::move(ary));
-        return *this;
-    }
-
-    // array (STL Containers) ------------------------------------------------
-
-    template<typename T, typename std::enable_if<detail::conjunction<
-            detail::negation<std::is_same<T, array_type>>,
-            detail::is_container<T>
-        >::value, std::nullptr_t>::type = nullptr>
-    basic_value(const T& list)
-        : type_(value_t::array),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        static_assert(std::is_convertible<typename T::value_type, value_type>::value,
-            "elements of a container should be convertible to toml::value");
-
-        array_type ary(list.size());
-        std::copy(list.begin(), list.end(), ary.begin());
-        assigner(this->array_, std::move(ary));
-    }
-    template<typename T, typename std::enable_if<detail::conjunction<
-            detail::negation<std::is_same<T, array_type>>,
-            detail::is_container<T>
-        >::value, std::nullptr_t>::type = nullptr>
-    basic_value(const T& list, std::vector<std::string> com)
-        : type_(value_t::array),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        static_assert(std::is_convertible<typename T::value_type, value_type>::value,
-            "elements of a container should be convertible to toml::value");
-
-        array_type ary(list.size());
-        std::copy(list.begin(), list.end(), ary.begin());
-        assigner(this->array_, std::move(ary));
-    }
-    template<typename T, typename std::enable_if<detail::conjunction<
-            detail::negation<std::is_same<T, array_type>>,
-            detail::is_container<T>
-        >::value, std::nullptr_t>::type = nullptr>
-    basic_value& operator=(const T& list)
-    {
-        static_assert(std::is_convertible<typename T::value_type, value_type>::value,
-            "elements of a container should be convertible to toml::value");
-
-        this->cleanup();
-        this->type_ = value_t::array;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-
-        array_type ary(list.size());
-        std::copy(list.begin(), list.end(), ary.begin());
-        assigner(this->array_, std::move(ary));
-        return *this;
-    }
-
-    // table ================================================================
-
-    basic_value(const table_type& tab)
-        : type_(value_t::table),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        assigner(this->table_, tab);
-    }
-    basic_value(const table_type& tab, std::vector<std::string> com)
-        : type_(value_t::table),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        assigner(this->table_, tab);
-    }
-    basic_value& operator=(const table_type& tab)
-    {
-        this->cleanup();
-        this->type_ = value_t::table;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-        assigner(this->table_, tab);
-        return *this;
-    }
-
-    // initializer-list ------------------------------------------------------
-
-    basic_value(std::initializer_list<std::pair<key, basic_value>> list)
-        : type_(value_t::table),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        table_type tab;
-        for(const auto& elem : list) {tab[elem.first] = elem.second;}
-        assigner(this->table_, std::move(tab));
-    }
-
-    basic_value(std::initializer_list<std::pair<key, basic_value>> list,
-                std::vector<std::string> com)
-        : type_(value_t::table),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        table_type tab;
-        for(const auto& elem : list) {tab[elem.first] = elem.second;}
-        assigner(this->table_, std::move(tab));
-    }
-    basic_value& operator=(std::initializer_list<std::pair<key, basic_value>> list)
-    {
-        this->cleanup();
-        this->type_ = value_t::table;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-
-        table_type tab;
-        for(const auto& elem : list) {tab[elem.first] = elem.second;}
-        assigner(this->table_, std::move(tab));
-        return *this;
-    }
-
-    // other table-like -----------------------------------------------------
-
-    template<typename Map, typename std::enable_if<detail::conjunction<
-            detail::negation<std::is_same<Map, table_type>>,
-            detail::is_map<Map>
-        >::value, std::nullptr_t>::type = nullptr>
-    basic_value(const Map& mp)
-        : type_(value_t::table),
-          region_info_(std::make_shared<region_base>(region_base{}))
-    {
-        table_type tab;
-        for(const auto& elem : mp) {tab[elem.first] = elem.second;}
-        assigner(this->table_, std::move(tab));
-    }
-    template<typename Map, typename std::enable_if<detail::conjunction<
-            detail::negation<std::is_same<Map, table_type>>,
-            detail::is_map<Map>
-        >::value, std::nullptr_t>::type = nullptr>
-    basic_value(const Map& mp, std::vector<std::string> com)
-        : type_(value_t::table),
-          region_info_(std::make_shared<region_base>(region_base{})),
-          comments_(std::move(com))
-    {
-        table_type tab;
-        for(const auto& elem : mp) {tab[elem.first] = elem.second;}
-        assigner(this->table_, std::move(tab));
-    }
-    template<typename Map, typename std::enable_if<detail::conjunction<
-            detail::negation<std::is_same<Map, table_type>>,
-            detail::is_map<Map>
-        >::value, std::nullptr_t>::type = nullptr>
-    basic_value& operator=(const Map& mp)
-    {
-        this->cleanup();
-        this->type_ = value_t::table;
-        this->region_info_ = std::make_shared<region_base>(region_base{});
-
-        table_type tab;
-        for(const auto& elem : mp) {tab[elem.first] = elem.second;}
-        assigner(this->table_, std::move(tab));
-        return *this;
-    }
-
-    // user-defined =========================================================
-
-    // convert using into_toml() method -------------------------------------
-
-    template<typename T, typename std::enable_if<
-        detail::has_into_toml_method<T>::value, std::nullptr_t>::type = nullptr>
-    basic_value(const T& ud): basic_value(ud.into_toml()) {}
-
-    template<typename T, typename std::enable_if<
-        detail::has_into_toml_method<T>::value, std::nullptr_t>::type = nullptr>
-    basic_value(const T& ud, std::vector<std::string> com)
-        : basic_value(ud.into_toml(), std::move(com))
-    {}
-    template<typename T, typename std::enable_if<
-        detail::has_into_toml_method<T>::value, std::nullptr_t>::type = nullptr>
-    basic_value& operator=(const T& ud)
-    {
-        *this = ud.into_toml();
-        return *this;
-    }
-
-    // convert using into<T> struct -----------------------------------------
-
-    template<typename T, std::size_t S = sizeof(::toml::into<T>)>
-    basic_value(const T& ud): basic_value(::toml::into<T>::into_toml(ud)) {}
-    template<typename T, std::size_t S = sizeof(::toml::into<T>)>
-    basic_value(const T& ud, std::vector<std::string> com)
-        : basic_value(::toml::into<T>::into_toml(ud), std::move(com))
-    {}
-    template<typename T, std::size_t S = sizeof(::toml::into<T>)>
-    basic_value& operator=(const T& ud)
-    {
-        *this = ::toml::into<T>::into_toml(ud);
-        return *this;
-    }
-
-    // for internal use ------------------------------------------------------
-    //
-    // Those constructors take detail::region that contains parse result.
-
-    basic_value(boolean b, detail::region reg, std::vector<std::string> cm)
-        : type_(value_t::boolean),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->boolean_, b);
-    }
-    template<typename T, typename std::enable_if<
-        detail::conjunction<
-            std::is_integral<T>, detail::negation<std::is_same<T, boolean>>
-        >::value, std::nullptr_t>::type = nullptr>
-    basic_value(T i, detail::region reg, std::vector<std::string> cm)
-        : type_(value_t::integer),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->integer_, static_cast<integer>(i));
-    }
-    template<typename T, typename std::enable_if<
-        std::is_floating_point<T>::value, std::nullptr_t>::type = nullptr>
-    basic_value(T f, detail::region reg, std::vector<std::string> cm)
-        : type_(value_t::floating),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->floating_, static_cast<floating>(f));
-    }
-    basic_value(toml::string s, detail::region reg,
-                std::vector<std::string> cm)
-        : type_(value_t::string),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->string_, std::move(s));
-    }
-    basic_value(const local_date& ld, detail::region reg,
-                std::vector<std::string> cm)
-        : type_(value_t::local_date),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->local_date_, ld);
-    }
-    basic_value(const local_time& lt, detail::region reg,
-                std::vector<std::string> cm)
-        : type_(value_t::local_time),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->local_time_, lt);
-    }
-    basic_value(const local_datetime& ldt, detail::region reg,
-                std::vector<std::string> cm)
-        : type_(value_t::local_datetime),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->local_datetime_, ldt);
-    }
-    basic_value(const offset_datetime& odt, detail::region reg,
-                std::vector<std::string> cm)
-        : type_(value_t::offset_datetime),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->offset_datetime_, odt);
-    }
-    basic_value(const array_type& ary, detail::region reg,
-                std::vector<std::string> cm)
-        : type_(value_t::array),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->array_, ary);
-    }
-    basic_value(const table_type& tab, detail::region reg,
-                std::vector<std::string> cm)
-        : type_(value_t::table),
-          region_info_(std::make_shared<detail::region>(std::move(reg))),
-          comments_(std::move(cm))
-    {
-        assigner(this->table_, tab);
-    }
-
-    template<typename T, typename std::enable_if<
-        detail::is_exact_toml_type<T, value_type>::value,
-        std::nullptr_t>::type = nullptr>
-    basic_value(std::pair<T, detail::region> parse_result, std::vector<std::string> com)
-        : basic_value(std::move(parse_result.first),
-                      std::move(parse_result.second),
-                      std::move(com))
-    {}
-
-    // type checking and casting ============================================
-
-    template<typename T, typename std::enable_if<
-        detail::is_exact_toml_type<T, value_type>::value,
-        std::nullptr_t>::type = nullptr>
-    bool is() const noexcept
-    {
-        return detail::type_to_enum<T, value_type>::value == this->type_;
-    }
-    bool is(value_t t) const noexcept {return t == this->type_;}
-
-    bool is_uninitialized()   const noexcept {return this->is(value_t::empty          );}
-    bool is_boolean()         const noexcept {return this->is(value_t::boolean        );}
-    bool is_integer()         const noexcept {return this->is(value_t::integer        );}
-    bool is_floating()        const noexcept {return this->is(value_t::floating       );}
-    bool is_string()          const noexcept {return this->is(value_t::string         );}
-    bool is_offset_datetime() const noexcept {return this->is(value_t::offset_datetime);}
-    bool is_local_datetime()  const noexcept {return this->is(value_t::local_datetime );}
-    bool is_local_date()      const noexcept {return this->is(value_t::local_date     );}
-    bool is_local_time()      const noexcept {return this->is(value_t::local_time     );}
-    bool is_array()           const noexcept {return this->is(value_t::array          );}
-    bool is_table()           const noexcept {return this->is(value_t::table          );}
-
-    value_t type() const noexcept {return type_;}
-
-    template<value_t T>
-    typename detail::enum_to_type<T, value_type>::type&       cast() &
-    {
-        if(this->type_ != T)
-        {
-            detail::throw_bad_cast<T>("toml::value::cast: ", this->type_, *this);
-        }
-        return detail::switch_cast<T>::invoke(*this);
-    }
-    template<value_t T>
-    typename detail::enum_to_type<T, value_type>::type const& cast() const&
-    {
-        if(this->type_ != T)
-        {
-            detail::throw_bad_cast<T>("toml::value::cast: ", this->type_, *this);
-        }
-        return detail::switch_cast<T>::invoke(*this);
-    }
-    template<value_t T>
-    typename detail::enum_to_type<T, value_type>::type&&      cast() &&
-    {
-        if(this->type_ != T)
-        {
-            detail::throw_bad_cast<T>("toml::value::cast: ", this->type_, *this);
-        }
-        return detail::switch_cast<T>::invoke(std::move(*this));
-    }
-
-    // ------------------------------------------------------------------------
-    // nothrow version
-
-    boolean         const& as_boolean        (const std::nothrow_t&) const& noexcept {return this->boolean_;}
-    integer         const& as_integer        (const std::nothrow_t&) const& noexcept {return this->integer_;}
-    floating        const& as_floating       (const std::nothrow_t&) const& noexcept {return this->floating_;}
-    string          const& as_string         (const std::nothrow_t&) const& noexcept {return this->string_;}
-    offset_datetime const& as_offset_datetime(const std::nothrow_t&) const& noexcept {return this->offset_datetime_;}
-    local_datetime  const& as_local_datetime (const std::nothrow_t&) const& noexcept {return this->local_datetime_;}
-    local_date      const& as_local_date     (const std::nothrow_t&) const& noexcept {return this->local_date_;}
-    local_time      const& as_local_time     (const std::nothrow_t&) const& noexcept {return this->local_time_;}
-    array_type      const& as_array          (const std::nothrow_t&) const& noexcept {return this->array_.value();}
-    table_type      const& as_table          (const std::nothrow_t&) const& noexcept {return this->table_.value();}
-
-    boolean        & as_boolean        (const std::nothrow_t&) & noexcept {return this->boolean_;}
-    integer        & as_integer        (const std::nothrow_t&) & noexcept {return this->integer_;}
-    floating       & as_floating       (const std::nothrow_t&) & noexcept {return this->floating_;}
-    string         & as_string         (const std::nothrow_t&) & noexcept {return this->string_;}
-    offset_datetime& as_offset_datetime(const std::nothrow_t&) & noexcept {return this->offset_datetime_;}
-    local_datetime & as_local_datetime (const std::nothrow_t&) & noexcept {return this->local_datetime_;}
-    local_date     & as_local_date     (const std::nothrow_t&) & noexcept {return this->local_date_;}
-    local_time     & as_local_time     (const std::nothrow_t&) & noexcept {return this->local_time_;}
-    array_type     & as_array          (const std::nothrow_t&) & noexcept {return this->array_.value();}
-    table_type     & as_table          (const std::nothrow_t&) & noexcept {return this->table_.value();}
-
-    boolean        && as_boolean        (const std::nothrow_t&) && noexcept {return std::move(this->boolean_);}
-    integer        && as_integer        (const std::nothrow_t&) && noexcept {return std::move(this->integer_);}
-    floating       && as_floating       (const std::nothrow_t&) && noexcept {return std::move(this->floating_);}
-    string         && as_string         (const std::nothrow_t&) && noexcept {return std::move(this->string_);}
-    offset_datetime&& as_offset_datetime(const std::nothrow_t&) && noexcept {return std::move(this->offset_datetime_);}
-    local_datetime && as_local_datetime (const std::nothrow_t&) && noexcept {return std::move(this->local_datetime_);}
-    local_date     && as_local_date     (const std::nothrow_t&) && noexcept {return std::move(this->local_date_);}
-    local_time     && as_local_time     (const std::nothrow_t&) && noexcept {return std::move(this->local_time_);}
-    array_type     && as_array          (const std::nothrow_t&) && noexcept {return std::move(this->array_.value());}
-    table_type     && as_table          (const std::nothrow_t&) && noexcept {return std::move(this->table_.value());}
-
-    // ========================================================================
-    // throw version
-    // ------------------------------------------------------------------------
-    // const reference {{{
-
-    boolean const& as_boolean() const&
-    {
-        if(this->type_ != value_t::boolean)
-        {
-            detail::throw_bad_cast<value_t::boolean>(
-                    "toml::value::as_boolean(): ", this->type_, *this);
-        }
-        return this->boolean_;
-    }
-    integer const& as_integer() const&
-    {
-        if(this->type_ != value_t::integer)
-        {
-            detail::throw_bad_cast<value_t::integer>(
-                    "toml::value::as_integer(): ", this->type_, *this);
-        }
-        return this->integer_;
-    }
-    floating const& as_floating() const&
-    {
-        if(this->type_ != value_t::floating)
-        {
-            detail::throw_bad_cast<value_t::floating>(
-                    "toml::value::as_floating(): ", this->type_, *this);
-        }
-        return this->floating_;
-    }
-    string const& as_string() const&
-    {
-        if(this->type_ != value_t::string)
-        {
-            detail::throw_bad_cast<value_t::string>(
-                    "toml::value::as_string(): ", this->type_, *this);
-        }
-        return this->string_;
-    }
-    offset_datetime const& as_offset_datetime() const&
-    {
-        if(this->type_ != value_t::offset_datetime)
-        {
-            detail::throw_bad_cast<value_t::offset_datetime>(
-                    "toml::value::as_offset_datetime(): ", this->type_, *this);
-        }
-        return this->offset_datetime_;
-    }
-    local_datetime const& as_local_datetime() const&
-    {
-        if(this->type_ != value_t::local_datetime)
-        {
-            detail::throw_bad_cast<value_t::local_datetime>(
-                    "toml::value::as_local_datetime(): ", this->type_, *this);
-        }
-        return this->local_datetime_;
-    }
-    local_date const& as_local_date() const&
-    {
-        if(this->type_ != value_t::local_date)
-        {
-            detail::throw_bad_cast<value_t::local_date>(
-                    "toml::value::as_local_date(): ", this->type_, *this);
-        }
-        return this->local_date_;
-    }
-    local_time const& as_local_time() const&
-    {
-        if(this->type_ != value_t::local_time)
-        {
-            detail::throw_bad_cast<value_t::local_time>(
-                    "toml::value::as_local_time(): ", this->type_, *this);
-        }
-        return this->local_time_;
-    }
-    array_type const& as_array() const&
-    {
-        if(this->type_ != value_t::array)
-        {
-            detail::throw_bad_cast<value_t::array>(
-                    "toml::value::as_array(): ", this->type_, *this);
-        }
-        return this->array_.value();
-    }
-    table_type const& as_table() const&
-    {
-        if(this->type_ != value_t::table)
-        {
-            detail::throw_bad_cast<value_t::table>(
-                    "toml::value::as_table(): ", this->type_, *this);
-        }
-        return this->table_.value();
-    }
-    // }}}
-    // ------------------------------------------------------------------------
-    // nonconst reference {{{
-
-    boolean & as_boolean() &
-    {
-        if(this->type_ != value_t::boolean)
-        {
-            detail::throw_bad_cast<value_t::boolean>(
-                    "toml::value::as_boolean(): ", this->type_, *this);
-        }
-        return this->boolean_;
-    }
-    integer & as_integer() &
-    {
-        if(this->type_ != value_t::integer)
-        {
-            detail::throw_bad_cast<value_t::integer>(
-                    "toml::value::as_integer(): ", this->type_, *this);
-        }
-        return this->integer_;
-    }
-    floating & as_floating() &
-    {
-        if(this->type_ != value_t::floating)
-        {
-            detail::throw_bad_cast<value_t::floating>(
-                    "toml::value::as_floating(): ", this->type_, *this);
-        }
-        return this->floating_;
-    }
-    string & as_string() &
-    {
-        if(this->type_ != value_t::string)
-        {
-            detail::throw_bad_cast<value_t::string>(
-                    "toml::value::as_string(): ", this->type_, *this);
-        }
-        return this->string_;
-    }
-    offset_datetime & as_offset_datetime() &
-    {
-        if(this->type_ != value_t::offset_datetime)
-        {
-            detail::throw_bad_cast<value_t::offset_datetime>(
-                    "toml::value::as_offset_datetime(): ", this->type_, *this);
-        }
-        return this->offset_datetime_;
-    }
-    local_datetime & as_local_datetime() &
-    {
-        if(this->type_ != value_t::local_datetime)
-        {
-            detail::throw_bad_cast<value_t::local_datetime>(
-                    "toml::value::as_local_datetime(): ", this->type_, *this);
-        }
-        return this->local_datetime_;
-    }
-    local_date & as_local_date() &
-    {
-        if(this->type_ != value_t::local_date)
-        {
-            detail::throw_bad_cast<value_t::local_date>(
-                    "toml::value::as_local_date(): ", this->type_, *this);
-        }
-        return this->local_date_;
-    }
-    local_time & as_local_time() &
-    {
-        if(this->type_ != value_t::local_time)
-        {
-            detail::throw_bad_cast<value_t::local_time>(
-                    "toml::value::as_local_time(): ", this->type_, *this);
-        }
-        return this->local_time_;
-    }
-    array_type & as_array() &
-    {
-        if(this->type_ != value_t::array)
-        {
-            detail::throw_bad_cast<value_t::array>(
-                    "toml::value::as_array(): ", this->type_, *this);
-        }
-        return this->array_.value();
-    }
-    table_type & as_table() &
-    {
-        if(this->type_ != value_t::table)
-        {
-            detail::throw_bad_cast<value_t::table>(
-                    "toml::value::as_table(): ", this->type_, *this);
-        }
-        return this->table_.value();
-    }
-
-    // }}}
-    // ------------------------------------------------------------------------
-    // rvalue reference {{{
-
-    boolean && as_boolean() &&
-    {
-        if(this->type_ != value_t::boolean)
-        {
-            detail::throw_bad_cast<value_t::boolean>(
-                    "toml::value::as_boolean(): ", this->type_, *this);
-        }
-        return std::move(this->boolean_);
-    }
-    integer && as_integer() &&
-    {
-        if(this->type_ != value_t::integer)
-        {
-            detail::throw_bad_cast<value_t::integer>(
-                    "toml::value::as_integer(): ", this->type_, *this);
-        }
-        return std::move(this->integer_);
-    }
-    floating && as_floating() &&
-    {
-        if(this->type_ != value_t::floating)
-        {
-            detail::throw_bad_cast<value_t::floating>(
-                    "toml::value::as_floating(): ", this->type_, *this);
-        }
-        return std::move(this->floating_);
-    }
-    string && as_string() &&
-    {
-        if(this->type_ != value_t::string)
-        {
-            detail::throw_bad_cast<value_t::string>(
-                    "toml::value::as_string(): ", this->type_, *this);
-        }
-        return std::move(this->string_);
-    }
-    offset_datetime && as_offset_datetime() &&
-    {
-        if(this->type_ != value_t::offset_datetime)
-        {
-            detail::throw_bad_cast<value_t::offset_datetime>(
-                    "toml::value::as_offset_datetime(): ", this->type_, *this);
-        }
-        return std::move(this->offset_datetime_);
-    }
-    local_datetime && as_local_datetime() &&
-    {
-        if(this->type_ != value_t::local_datetime)
-        {
-            detail::throw_bad_cast<value_t::local_datetime>(
-                    "toml::value::as_local_datetime(): ", this->type_, *this);
-        }
-        return std::move(this->local_datetime_);
-    }
-    local_date && as_local_date() &&
-    {
-        if(this->type_ != value_t::local_date)
-        {
-            detail::throw_bad_cast<value_t::local_date>(
-                    "toml::value::as_local_date(): ", this->type_, *this);
-        }
-        return std::move(this->local_date_);
-    }
-    local_time && as_local_time() &&
-    {
-        if(this->type_ != value_t::local_time)
-        {
-            detail::throw_bad_cast<value_t::local_time>(
-                    "toml::value::as_local_time(): ", this->type_, *this);
-        }
-        return std::move(this->local_time_);
-    }
-    array_type && as_array() &&
-    {
-        if(this->type_ != value_t::array)
-        {
-            detail::throw_bad_cast<value_t::array>(
-                    "toml::value::as_array(): ", this->type_, *this);
-        }
-        return std::move(this->array_.value());
-    }
-    table_type && as_table() &&
-    {
-        if(this->type_ != value_t::table)
-        {
-            detail::throw_bad_cast<value_t::table>(
-                    "toml::value::as_table(): ", this->type_, *this);
-        }
-        return std::move(this->table_.value());
-    }
-    // }}}
-
-    // accessors =============================================================
-    //
-    // may throw type_error or out_of_range
-    //
-    value_type&       at(const key& k)
-    {
-        if(!this->is_table())
-        {
-            detail::throw_bad_cast<value_t::table>(
-                "toml::value::at(key): ", this->type_, *this);
-        }
-        if(this->as_table(std::nothrow).count(k) == 0)
-        {
-            detail::throw_key_not_found_error(*this, k);
-        }
-        return this->as_table(std::nothrow).at(k);
-    }
-    value_type const& at(const key& k) const
-    {
-        if(!this->is_table())
-        {
-            detail::throw_bad_cast<value_t::table>(
-                "toml::value::at(key): ", this->type_, *this);
-        }
-        if(this->as_table(std::nothrow).count(k) == 0)
-        {
-            detail::throw_key_not_found_error(*this, k);
-        }
-        return this->as_table(std::nothrow).at(k);
-    }
-    value_type&       operator[](const key& k)
-    {
-        if(this->is_uninitialized())
-        {
-            *this = table_type{};
-        }
-        else if(!this->is_table()) // initialized, but not a table
-        {
-            detail::throw_bad_cast<value_t::table>(
-                "toml::value::operator[](key): ", this->type_, *this);
-        }
-        return this->as_table(std::nothrow)[k];
-    }
-
-    value_type&       at(const std::size_t idx)
-    {
-        if(!this->is_array())
-        {
-            detail::throw_bad_cast<value_t::array>(
-                "toml::value::at(idx): ", this->type_, *this);
-        }
-        if(this->as_array(std::nothrow).size() <= idx)
-        {
-            throw std::out_of_range(detail::format_underline(
-                "toml::value::at(idx): no element corresponding to the index", {
-                    {this->location(), concat_to_string("the length is ",
-                        this->as_array(std::nothrow).size(),
-                        ", and the specified index is ", idx)}
-                }));
-        }
-        return this->as_array().at(idx);
-    }
-    value_type const& at(const std::size_t idx) const
-    {
-        if(!this->is_array())
-        {
-            detail::throw_bad_cast<value_t::array>(
-                "toml::value::at(idx): ", this->type_, *this);
-        }
-        if(this->as_array(std::nothrow).size() <= idx)
-        {
-            throw std::out_of_range(detail::format_underline(
-                "toml::value::at(idx): no element corresponding to the index", {
-                    {this->location(), concat_to_string("the length is ",
-                        this->as_array(std::nothrow).size(),
-                        ", and the specified index is ", idx)}
-                }));
-        }
-        return this->as_array(std::nothrow).at(idx);
-    }
-
-    value_type&       operator[](const std::size_t idx) noexcept
-    {
-        // no check...
-        return this->as_array(std::nothrow)[idx];
-    }
-    value_type const& operator[](const std::size_t idx) const noexcept
-    {
-        // no check...
-        return this->as_array(std::nothrow)[idx];
-    }
-
-    void push_back(const value_type& x)
-    {
-        if(!this->is_array())
-        {
-            detail::throw_bad_cast<value_t::array>(
-                "toml::value::push_back(value): ", this->type_, *this);
-        }
-        this->as_array(std::nothrow).push_back(x);
-        return;
-    }
-    void push_back(value_type&& x)
-    {
-        if(!this->is_array())
-        {
-            detail::throw_bad_cast<value_t::array>(
-                "toml::value::push_back(value): ", this->type_, *this);
-        }
-        this->as_array(std::nothrow).push_back(std::move(x));
-        return;
-    }
-
-    template<typename ... Ts>
-    value_type& emplace_back(Ts&& ... args)
-    {
-        if(!this->is_array())
-        {
-            detail::throw_bad_cast<value_t::array>(
-                "toml::value::emplace_back(...): ", this->type_, *this);
-        }
-        this->as_array(std::nothrow).emplace_back(std::forward<Ts>(args) ...);
-        return this->as_array(std::nothrow).back();
-    }
-
-    std::size_t size() const
-    {
-        switch(this->type_)
-        {
-            case value_t::array:
-            {
-                return this->as_array(std::nothrow).size();
-            }
-            case value_t::table:
-            {
-                return this->as_table(std::nothrow).size();
-            }
-            case value_t::string:
-            {
-                return this->as_string(std::nothrow).str.size();
-            }
-            default:
-            {
-                throw type_error(detail::format_underline(
-                    "toml::value::size(): bad_cast to container types", {
-                        {this->location(),
-                         concat_to_string("the actual type is ", this->type_)}
-                    }), this->location());
-            }
-        }
-    }
-
-    std::size_t count(const key_type& k) const
-    {
-        if(!this->is_table())
-        {
-            detail::throw_bad_cast<value_t::table>(
-                "toml::value::count(key): ", this->type_, *this);
-        }
-        return this->as_table(std::nothrow).count(k);
-    }
-
-    bool contains(const key_type& k) const
-    {
-        if(!this->is_table())
-        {
-            detail::throw_bad_cast<value_t::table>(
-                "toml::value::contains(key): ", this->type_, *this);
-        }
-        return (this->as_table(std::nothrow).count(k) != 0);
-    }
-
-    source_location location() const
-    {
-        return source_location(this->region_info_.get());
-    }
-
-    comment_type const& comments() const noexcept {return this->comments_;}
-    comment_type&       comments()       noexcept {return this->comments_;}
-
-  private:
-
-    void cleanup() noexcept
-    {
-        switch(this->type_)
-        {
-            case value_t::string  : {string_.~string();       return;}
-            case value_t::array   : {array_.~array_storage(); return;}
-            case value_t::table   : {table_.~table_storage(); return;}
-            default              : return;
-        }
-    }
-
-    // for error messages
-    template<typename Value>
-    friend region_base const* detail::get_region(const Value& v);
-
-    template<typename Value>
-    friend void detail::change_region(Value& v, detail::region reg);
-
-  private:
-
-    using array_storage = detail::storage<array_type>;
-    using table_storage = detail::storage<table_type>;
-
-    value_t type_;
-    union
-    {
-        boolean         boolean_;
-        integer         integer_;
-        floating        floating_;
-        string          string_;
-        offset_datetime offset_datetime_;
-        local_datetime  local_datetime_;
-        local_date      local_date_;
-        local_time      local_time_;
-        array_storage   array_;
-        table_storage   table_;
-    };
-    std::shared_ptr<region_base> region_info_;
-    comment_type                 comments_;
-};
-
-// default toml::value and default array/table.
-// TOML11_DEFAULT_COMMENT_STRATEGY is defined in comments.hpp
-using value = basic_value<TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>;
-using array = typename value::array_type;
-using table = typename value::table_type;
-
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-inline bool
-operator==(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
-{
-    if(lhs.type()     != rhs.type())     {return false;}
-    if(lhs.comments() != rhs.comments()) {return false;}
-
-    switch(lhs.type())
-    {
-        case value_t::boolean  :
-        {
-            return lhs.as_boolean() == rhs.as_boolean();
-        }
-        case value_t::integer  :
-        {
-            return lhs.as_integer() == rhs.as_integer();
-        }
-        case value_t::floating :
-        {
-            return lhs.as_floating() == rhs.as_floating();
-        }
-        case value_t::string   :
-        {
-            return lhs.as_string() == rhs.as_string();
-        }
-        case value_t::offset_datetime:
-        {
-            return lhs.as_offset_datetime() == rhs.as_offset_datetime();
-        }
-        case value_t::local_datetime:
-        {
-            return lhs.as_local_datetime() == rhs.as_local_datetime();
-        }
-        case value_t::local_date:
-        {
-            return lhs.as_local_date() == rhs.as_local_date();
-        }
-        case value_t::local_time:
-        {
-            return lhs.as_local_time() == rhs.as_local_time();
-        }
-        case value_t::array    :
-        {
-            return lhs.as_array() == rhs.as_array();
-        }
-        case value_t::table    :
-        {
-            return lhs.as_table() == rhs.as_table();
-        }
-        case value_t::empty    : {return true; }
-        default:                 {return false;}
-    }
-}
-
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-inline bool operator!=(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
-{
-    return !(lhs == rhs);
-}
-
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-typename std::enable_if<detail::conjunction<
-    detail::is_comparable<typename basic_value<C, T, A>::array_type>,
-    detail::is_comparable<typename basic_value<C, T, A>::table_type>
-    >::value, bool>::type
-operator<(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
-{
-    if(lhs.type() != rhs.type()){return (lhs.type() < rhs.type());}
-    switch(lhs.type())
-    {
-        case value_t::boolean  :
-        {
-            return lhs.as_boolean() <  rhs.as_boolean() ||
-                  (lhs.as_boolean() == rhs.as_boolean() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::integer  :
-        {
-            return lhs.as_integer() <  rhs.as_integer() ||
-                  (lhs.as_integer() == rhs.as_integer() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::floating :
-        {
-            return lhs.as_floating() <  rhs.as_floating() ||
-                  (lhs.as_floating() == rhs.as_floating() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::string   :
-        {
-            return lhs.as_string() <  rhs.as_string() ||
-                  (lhs.as_string() == rhs.as_string() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::offset_datetime:
-        {
-            return lhs.as_offset_datetime() <  rhs.as_offset_datetime() ||
-                  (lhs.as_offset_datetime() == rhs.as_offset_datetime() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::local_datetime:
-        {
-            return lhs.as_local_datetime() <  rhs.as_local_datetime() ||
-                  (lhs.as_local_datetime() == rhs.as_local_datetime() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::local_date:
-        {
-            return lhs.as_local_date() <  rhs.as_local_date() ||
-                  (lhs.as_local_date() == rhs.as_local_date() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::local_time:
-        {
-            return lhs.as_local_time() <  rhs.as_local_time() ||
-                  (lhs.as_local_time() == rhs.as_local_time() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::array    :
-        {
-            return lhs.as_array() <  rhs.as_array() ||
-                  (lhs.as_array() == rhs.as_array() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::table    :
-        {
-            return lhs.as_table() <  rhs.as_table() ||
-                  (lhs.as_table() == rhs.as_table() &&
-                   lhs.comments() < rhs.comments());
-        }
-        case value_t::empty    :
-        {
-            return lhs.comments() < rhs.comments();
-        }
-        default:
-        {
-            return lhs.comments() < rhs.comments();
-        }
-    }
-}
-
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-typename std::enable_if<detail::conjunction<
-    detail::is_comparable<typename basic_value<C, T, A>::array_type>,
-    detail::is_comparable<typename basic_value<C, T, A>::table_type>
-    >::value, bool>::type
-operator<=(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
-{
-    return (lhs < rhs) || (lhs == rhs);
-}
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-typename std::enable_if<detail::conjunction<
-    detail::is_comparable<typename basic_value<C, T, A>::array_type>,
-    detail::is_comparable<typename basic_value<C, T, A>::table_type>
-    >::value, bool>::type
-operator>(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
-{
-    return !(lhs <= rhs);
-}
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-typename std::enable_if<detail::conjunction<
-    detail::is_comparable<typename basic_value<C, T, A>::array_type>,
-    detail::is_comparable<typename basic_value<C, T, A>::table_type>
-    >::value, bool>::type
-operator>=(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
-{
-    return !(lhs < rhs);
-}
-
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-inline std::string format_error(const std::string& err_msg,
-        const basic_value<C, T, A>& v, const std::string& comment,
-        std::vector<std::string> hints = {},
-        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
-{
-    return detail::format_underline(err_msg, {{v.location(), comment}},
-                                    std::move(hints), colorize);
-}
-
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-inline std::string format_error(const std::string& err_msg,
-        const toml::basic_value<C, T, A>& v1, const std::string& comment1,
-        const toml::basic_value<C, T, A>& v2, const std::string& comment2,
-        std::vector<std::string> hints = {},
-        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
-{
-    return detail::format_underline(err_msg, {
-            {v1.location(), comment1}, {v2.location(), comment2}
-        }, std::move(hints), colorize);
-}
-
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-inline std::string format_error(const std::string& err_msg,
-        const toml::basic_value<C, T, A>& v1, const std::string& comment1,
-        const toml::basic_value<C, T, A>& v2, const std::string& comment2,
-        const toml::basic_value<C, T, A>& v3, const std::string& comment3,
-        std::vector<std::string> hints = {},
-        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
-{
-    return detail::format_underline(err_msg, {{v1.location(), comment1},
-            {v2.location(), comment2}, {v3.location(), comment3}
-        }, std::move(hints), colorize);
-}
-
-template<typename Visitor, typename C,
-         template<typename ...> class T, template<typename ...> class A>
-detail::return_type_of_t<Visitor, const toml::boolean&>
-visit(Visitor&& visitor, const toml::basic_value<C, T, A>& v)
-{
-    switch(v.type())
-    {
-        case value_t::boolean        : {return visitor(v.as_boolean        ());}
-        case value_t::integer        : {return visitor(v.as_integer        ());}
-        case value_t::floating       : {return visitor(v.as_floating       ());}
-        case value_t::string         : {return visitor(v.as_string         ());}
-        case value_t::offset_datetime: {return visitor(v.as_offset_datetime());}
-        case value_t::local_datetime : {return visitor(v.as_local_datetime ());}
-        case value_t::local_date     : {return visitor(v.as_local_date     ());}
-        case value_t::local_time     : {return visitor(v.as_local_time     ());}
-        case value_t::array          : {return visitor(v.as_array          ());}
-        case value_t::table          : {return visitor(v.as_table          ());}
-        case value_t::empty          : break;
-        default: break;
-    }
-    throw std::runtime_error(format_error("[error] toml::visit: toml::basic_value "
-            "does not have any valid basic_value.", v, "here"));
-}
-
-template<typename Visitor, typename C,
-         template<typename ...> class T, template<typename ...> class A>
-detail::return_type_of_t<Visitor, toml::boolean&>
-visit(Visitor&& visitor, toml::basic_value<C, T, A>& v)
-{
-    switch(v.type())
-    {
-        case value_t::boolean        : {return visitor(v.as_boolean        ());}
-        case value_t::integer        : {return visitor(v.as_integer        ());}
-        case value_t::floating       : {return visitor(v.as_floating       ());}
-        case value_t::string         : {return visitor(v.as_string         ());}
-        case value_t::offset_datetime: {return visitor(v.as_offset_datetime());}
-        case value_t::local_datetime : {return visitor(v.as_local_datetime ());}
-        case value_t::local_date     : {return visitor(v.as_local_date     ());}
-        case value_t::local_time     : {return visitor(v.as_local_time     ());}
-        case value_t::array          : {return visitor(v.as_array          ());}
-        case value_t::table          : {return visitor(v.as_table          ());}
-        case value_t::empty          : break;
-        default: break;
-    }
-    throw std::runtime_error(format_error("[error] toml::visit: toml::basic_value "
-            "does not have any valid basic_value.", v, "here"));
-}
-
-template<typename Visitor, typename C,
-         template<typename ...> class T, template<typename ...> class A>
-detail::return_type_of_t<Visitor, toml::boolean&&>
-visit(Visitor&& visitor, toml::basic_value<C, T, A>&& v)
-{
-    switch(v.type())
-    {
-        case value_t::boolean        : {return visitor(std::move(v.as_boolean        ()));}
-        case value_t::integer        : {return visitor(std::move(v.as_integer        ()));}
-        case value_t::floating       : {return visitor(std::move(v.as_floating       ()));}
-        case value_t::string         : {return visitor(std::move(v.as_string         ()));}
-        case value_t::offset_datetime: {return visitor(std::move(v.as_offset_datetime()));}
-        case value_t::local_datetime : {return visitor(std::move(v.as_local_datetime ()));}
-        case value_t::local_date     : {return visitor(std::move(v.as_local_date     ()));}
-        case value_t::local_time     : {return visitor(std::move(v.as_local_time     ()));}
-        case value_t::array          : {return visitor(std::move(v.as_array          ()));}
-        case value_t::table          : {return visitor(std::move(v.as_table          ()));}
-        case value_t::empty          : break;
-        default: break;
-    }
-    throw std::runtime_error(format_error("[error] toml::visit: toml::basic_value "
-            "does not have any valid basic_value.", v, "here"));
-}
-
-}// toml
-#endif// TOML11_VALUE
diff --git a/src/frontend/qt_sdl/toml/toml/version.hpp b/src/frontend/qt_sdl/toml/toml/version.hpp
deleted file mode 100644
index 9cbfa39b..00000000
--- a/src/frontend/qt_sdl/toml/toml/version.hpp
+++ /dev/null
@@ -1,42 +0,0 @@
-#ifndef TOML11_VERSION_HPP
-#define TOML11_VERSION_HPP
-
-// This file checks C++ version.
-
-#ifndef __cplusplus
-#    error "__cplusplus is not defined"
-#endif
-
-// Since MSVC does not define `__cplusplus` correctly unless you pass
-// `/Zc:__cplusplus` when compiling, the workaround macros are added.
-// Those enables you to define version manually or to use MSVC specific
-// version macro automatically.
-//
-// The value of `__cplusplus` macro is defined in the C++ standard spec, but
-// MSVC ignores the value, maybe because of backward compatibility. Instead,
-// MSVC defines _MSVC_LANG that has the same value as __cplusplus defined in
-// the C++ standard. First we check the manual version definition, and then
-// we check if _MSVC_LANG is defined. If neither, use normal `__cplusplus`.
-//
-// FYI: https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-170
-//      https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-170
-//
-#if   defined(TOML11_ENFORCE_CXX11)
-#  define TOML11_CPLUSPLUS_STANDARD_VERSION 201103L
-#elif defined(TOML11_ENFORCE_CXX14)
-#  define TOML11_CPLUSPLUS_STANDARD_VERSION 201402L
-#elif defined(TOML11_ENFORCE_CXX17)
-#  define TOML11_CPLUSPLUS_STANDARD_VERSION 201703L
-#elif defined(TOML11_ENFORCE_CXX20)
-#  define TOML11_CPLUSPLUS_STANDARD_VERSION 202002L
-#elif defined(_MSVC_LANG) && defined(_MSC_VER) && 1910 <= _MSC_VER
-#  define TOML11_CPLUSPLUS_STANDARD_VERSION _MSVC_LANG
-#else
-#  define TOML11_CPLUSPLUS_STANDARD_VERSION __cplusplus
-#endif
-
-#if TOML11_CPLUSPLUS_STANDARD_VERSION < 201103L && _MSC_VER < 1900
-#    error "toml11 requires C++11 or later."
-#endif
-
-#endif// TOML11_VERSION_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/color.hpp b/src/frontend/qt_sdl/toml/toml11/color.hpp
new file mode 100644
index 00000000..d40d7f80
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/color.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_COLOR_HPP
+#define TOML11_COLOR_HPP
+
+#include "fwd/color_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/color_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_COLOR_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/comments.hpp b/src/frontend/qt_sdl/toml/toml11/comments.hpp
new file mode 100644
index 00000000..4697f4e3
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/comments.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_COMMENTS_HPP
+#define TOML11_COMMENTS_HPP
+
+#include "fwd/comments_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/comments_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_COMMENTS_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/compat.hpp b/src/frontend/qt_sdl/toml/toml11/compat.hpp
new file mode 100644
index 00000000..3308a32d
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/compat.hpp
@@ -0,0 +1,751 @@
+#ifndef TOML11_COMPAT_HPP
+#define TOML11_COMPAT_HPP
+
+#include "version.hpp"
+
+#include <algorithm>
+#include <iterator>
+#include <memory>
+#include <string>
+#include <type_traits>
+
+#include <cassert>
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX20_VALUE
+#  if __has_include(<bit>)
+#    include <bit>
+#  endif
+#endif
+
+#include <cstring>
+
+// ----------------------------------------------------------------------------
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX14_VALUE
+#  if __has_cpp_attribute(deprecated)
+#    define TOML11_HAS_ATTR_DEPRECATED 1
+#  endif
+#endif
+
+#if defined(TOML11_HAS_ATTR_DEPRECATED)
+#  define TOML11_DEPRECATED(msg) [[deprecated(msg)]]
+#elif defined(__GNUC__)
+#  define TOML11_DEPRECATED(msg) __attribute__((deprecated(msg)))
+#elif defined(_MSC_VER)
+#  define TOML11_DEPRECATED(msg) __declspec(deprecated(msg))
+#else
+#  define TOML11_DEPRECATED(msg)
+#endif
+
+// ----------------------------------------------------------------------------
+
+#if defined(__cpp_if_constexpr)
+#  if __cpp_if_constexpr >= 201606L
+#    define TOML11_HAS_CONSTEXPR_IF 1
+#  endif
+#endif
+
+#if defined(TOML11_HAS_CONSTEXPR_IF)
+#  define TOML11_CONSTEXPR_IF if constexpr
+#else
+#  define TOML11_CONSTEXPR_IF if
+#endif
+
+// ----------------------------------------------------------------------------
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX14_VALUE
+#  if defined(__cpp_lib_make_unique)
+#    if __cpp_lib_make_unique >= 201304L
+#      define TOML11_HAS_STD_MAKE_UNIQUE 1
+#    endif
+#  endif
+#endif
+
+namespace toml
+{
+namespace cxx
+{
+
+#if defined(TOML11_HAS_STD_MAKE_UNIQUE)
+
+using std::make_unique;
+
+#else
+
+template<typename T, typename ... Ts>
+std::unique_ptr<T> make_unique(Ts&& ... args)
+{
+    return std::unique_ptr<T>(new T(std::forward<Ts>(args)...));
+}
+
+#endif // TOML11_HAS_STD_MAKE_UNIQUE
+
+} // cxx
+} // toml
+
+// ---------------------------------------------------------------------------
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX14_VALUE
+#  if defined(__cpp_lib_make_reverse_iterator)
+#    if __cpp_lib_make_reverse_iterator >= 201402L
+#      define TOML11_HAS_STD_MAKE_REVERSE_ITERATOR 1
+#    endif
+#  endif
+#endif
+
+namespace toml
+{
+namespace cxx
+{
+# if defined(TOML11_HAS_STD_MAKE_REVERSE_ITERATOR)
+
+using std::make_reverse_iterator;
+
+#else
+
+template<typename Iterator>
+std::reverse_iterator<Iterator> make_reverse_iterator(Iterator iter)
+{
+    return std::reverse_iterator<Iterator>(iter);
+}
+
+#endif // TOML11_HAS_STD_MAKE_REVERSE_ITERATOR
+
+} // cxx
+} // toml
+
+// ---------------------------------------------------------------------------
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX20_VALUE
+#  if defined(__cpp_lib_clamp)
+#    if __cpp_lib_clamp >= 201603L
+#      define TOML11_HAS_STD_CLAMP 1
+#    endif
+#  endif
+#endif
+
+namespace toml
+{
+namespace cxx
+{
+#if defined(TOML11_HAS_STD_CLAMP)
+
+using std::clamp;
+
+#else
+
+template<typename T>
+T clamp(const T& x, const T& low, const T& high) noexcept
+{
+    assert(low <= high);
+    return (std::min)((std::max)(x, low), high);
+}
+
+#endif // TOML11_HAS_STD_CLAMP
+
+} // cxx
+} // toml
+
+// ---------------------------------------------------------------------------
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX20_VALUE
+#  if defined(__cpp_lib_bit_cast)
+#    if __cpp_lib_bit_cast >= 201806L
+#      define TOML11_HAS_STD_BIT_CAST 1
+#    endif
+#  endif
+#endif
+
+namespace toml
+{
+namespace cxx
+{
+#if defined(TOML11_HAS_STD_BIT_CAST)
+
+using std::bit_cast;
+
+#else
+
+template<typename U, typename T>
+U bit_cast(const T& x) noexcept
+{
+    static_assert(sizeof(T) == sizeof(U), "");
+    static_assert(std::is_default_constructible<T>::value, "");
+
+    U z;
+    std::memcpy(reinterpret_cast<char*>(std::addressof(z)),
+                reinterpret_cast<const char*>(std::addressof(x)),
+                sizeof(T));
+
+    return z;
+}
+
+#endif // TOML11_HAS_STD_BIT_CAST
+
+} // cxx
+} // toml
+
+// ---------------------------------------------------------------------------
+// C++20 remove_cvref_t
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX20_VALUE
+#  if defined(__cpp_lib_remove_cvref)
+#    if __cpp_lib_remove_cvref >= 201711L
+#      define TOML11_HAS_STD_REMOVE_CVREF 1
+#    endif
+#  endif
+#endif
+
+namespace toml
+{
+namespace cxx
+{
+#if defined(TOML11_HAS_STD_REMOVE_CVREF)
+
+using std::remove_cvref;
+using std::remove_cvref_t;
+
+#else
+
+template<typename T>
+struct remove_cvref
+{
+    using type = typename std::remove_cv<
+        typename std::remove_reference<T>::type>::type;
+};
+
+template<typename T>
+using remove_cvref_t = typename remove_cvref<T>::type;
+
+#endif // TOML11_HAS_STD_REMOVE_CVREF
+
+} // cxx
+} // toml
+
+// ---------------------------------------------------------------------------
+// C++17 and/or/not
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX17_VALUE
+#  if defined(__cpp_lib_logical_traits)
+#    if __cpp_lib_logical_traits >= 201510L
+#      define TOML11_HAS_STD_CONJUNCTION 1
+#    endif
+#  endif
+#endif
+
+namespace toml
+{
+namespace cxx
+{
+#if defined(TOML11_HAS_STD_CONJUNCTION)
+
+using std::conjunction;
+using std::disjunction;
+using std::negation;
+
+#else
+
+template<typename ...> struct conjunction : std::true_type{};
+template<typename T>   struct conjunction<T> : T{};
+template<typename T, typename ... Ts>
+struct conjunction<T, Ts...> :
+    std::conditional<static_cast<bool>(T::value), conjunction<Ts...>, T>::type
+{};
+
+template<typename ...> struct disjunction : std::false_type{};
+template<typename T>   struct disjunction<T> : T {};
+template<typename T, typename ... Ts>
+struct disjunction<T, Ts...> :
+    std::conditional<static_cast<bool>(T::value), T, disjunction<Ts...>>::type
+{};
+
+template<typename T>
+struct negation : std::integral_constant<bool, !static_cast<bool>(T::value)>{};
+
+#endif // TOML11_HAS_STD_CONJUNCTION
+
+} // cxx
+} // toml
+
+// ---------------------------------------------------------------------------
+// C++14 index_sequence
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX14_VALUE
+#  if defined(__cpp_lib_integer_sequence)
+#    if __cpp_lib_integer_sequence >= 201304L
+#      define TOML11_HAS_STD_INTEGER_SEQUENCE 1
+#    endif
+#  endif
+#endif
+
+namespace toml
+{
+namespace cxx
+{
+#if defined(TOML11_HAS_STD_INTEGER_SEQUENCE)
+
+using std::index_sequence;
+using std::make_index_sequence;
+
+#else
+
+template<std::size_t ... Ns> struct index_sequence{};
+
+template<bool B, std::size_t N, typename T>
+struct double_index_sequence;
+
+template<std::size_t N, std::size_t ... Is>
+struct double_index_sequence<true, N, index_sequence<Is...>>
+{
+    using type = index_sequence<Is..., (Is+N)..., N*2>;
+};
+template<std::size_t N, std::size_t ... Is>
+struct double_index_sequence<false, N, index_sequence<Is...>>
+{
+    using type = index_sequence<Is..., (Is+N)...>;
+};
+
+template<std::size_t N>
+struct index_sequence_maker
+{
+    using type = typename double_index_sequence<
+            N % 2 == 1, N/2, typename index_sequence_maker<N/2>::type
+        >::type;
+};
+template<>
+struct index_sequence_maker<0>
+{
+    using type = index_sequence<>;
+};
+
+template<std::size_t N>
+using make_index_sequence = typename index_sequence_maker<N>::type;
+
+#endif // TOML11_HAS_STD_INTEGER_SEQUENCE
+
+} // cxx
+} // toml
+
+// ---------------------------------------------------------------------------
+// C++14 enable_if_t
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX14_VALUE
+#  if defined(__cpp_lib_transformation_trait_aliases)
+#    if __cpp_lib_transformation_trait_aliases >= 201304L
+#      define TOML11_HAS_STD_ENABLE_IF_T 1
+#    endif
+#  endif
+#endif
+
+namespace toml
+{
+namespace cxx
+{
+#if defined(TOML11_HAS_STD_ENABLE_IF_T)
+
+using std::enable_if_t;
+
+#else
+
+template<bool B, typename T>
+using enable_if_t = typename std::enable_if<B, T>::type;
+
+#endif // TOML11_HAS_STD_ENABLE_IF_T
+
+} // cxx
+} // toml
+
+// ---------------------------------------------------------------------------
+// return_type_of_t
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX17_VALUE
+#  if defined(__cpp_lib_is_invocable)
+#    if __cpp_lib_is_invocable >= 201703
+#      define TOML11_HAS_STD_INVOKE_RESULT 1
+#    endif
+#  endif
+#endif
+
+namespace toml
+{
+namespace cxx
+{
+#if defined(TOML11_HAS_STD_INVOKE_RESULT)
+
+template<typename F, typename ... Args>
+using return_type_of_t = std::invoke_result_t<F, Args...>;
+
+#else
+
+// result_of is deprecated after C++17
+template<typename F, typename ... Args>
+using return_type_of_t = typename std::result_of<F(Args...)>::type;
+
+#endif // TOML11_HAS_STD_INVOKE_RESULT
+
+} // cxx
+} // toml
+
+// ----------------------------------------------------------------------------
+// (subset of) source_location
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 202002L
+#  if __has_include(<source_location>)
+#    define TOML11_HAS_STD_SOURCE_LOCATION
+#  endif // has_include
+#endif // c++20
+
+#if ! defined(TOML11_HAS_STD_SOURCE_LOCATION)
+#  if defined(__GNUC__) && ! defined(__clang__)
+#    if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX14_VALUE
+#      if __has_include(<experimental/source_location>)
+#        define TOML11_HAS_EXPERIMENTAL_SOURCE_LOCATION
+#      endif
+#    endif
+#  endif // GNU g++
+#endif // not TOML11_HAS_STD_SOURCE_LOCATION
+
+#if ! defined(TOML11_HAS_STD_SOURCE_LOCATION) && ! defined(TOML11_HAS_EXPERIMENTAL_SOURCE_LOCATION)
+#  if defined(__GNUC__) && ! defined(__clang__)
+#    if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9))
+#      define TOML11_HAS_BUILTIN_FILE_LINE 1
+#      define TOML11_BUILTIN_LINE_TYPE int
+#    endif
+#  elif defined(__clang__) // clang 9.0.0 implements builtin_FILE/LINE
+#    if __has_builtin(__builtin_FILE) && __has_builtin(__builtin_LINE)
+#      define TOML11_HAS_BUILTIN_FILE_LINE 1
+#      define TOML11_BUILTIN_LINE_TYPE unsigned int
+#    endif
+#  elif defined(_MSVC_LANG) && defined(_MSC_VER)
+#    if _MSC_VER > 1926
+#      define TOML11_HAS_BUILTIN_FILE_LINE 1
+#      define TOML11_BUILTIN_LINE_TYPE int
+#    endif
+#  endif
+#endif
+
+#if defined(TOML11_HAS_STD_SOURCE_LOCATION)
+#include <source_location>
+namespace toml
+{
+namespace cxx
+{
+using source_location = std::source_location;
+
+inline std::string to_string(const source_location& loc)
+{
+    return std::string(" at line ") + std::to_string(loc.line()) +
+           std::string(" in file ") + std::string(loc.file_name());
+}
+} // cxx
+} // toml
+#elif defined(TOML11_HAS_EXPERIMENTAL_SOURCE_LOCATION)
+#include <experimental/source_location>
+namespace toml
+{
+namespace cxx
+{
+using source_location = std::experimental::source_location;
+
+inline std::string to_string(const source_location& loc)
+{
+    return std::string(" at line ") + std::to_string(loc.line()) +
+           std::string(" in file ") + std::string(loc.file_name());
+}
+} // cxx
+} // toml
+#elif defined(TOML11_HAS_BUILTIN_FILE_LINE)
+namespace toml
+{
+namespace cxx
+{
+struct source_location
+{
+    using line_type = TOML11_BUILTIN_LINE_TYPE;
+    static source_location current(const line_type line = __builtin_LINE(),
+                                   const char*     file = __builtin_FILE())
+    {
+        return source_location(line, file);
+    }
+
+    source_location(const line_type line, const char* file)
+        : line_(line), file_name_(file)
+    {}
+
+    line_type   line()      const noexcept {return line_;}
+    const char* file_name() const noexcept {return file_name_;}
+
+  private:
+
+    line_type   line_;
+    const char* file_name_;
+};
+
+inline std::string to_string(const source_location& loc)
+{
+    return std::string(" at line ") + std::to_string(loc.line()) +
+           std::string(" in file ") + std::string(loc.file_name());
+}
+} // cxx
+} // toml
+#else // no builtin
+namespace toml
+{
+namespace cxx
+{
+struct source_location
+{
+    static source_location current() { return source_location{}; }
+};
+
+inline std::string to_string(const source_location&)
+{
+    return std::string("");
+}
+} // cxx
+} // toml
+#endif // TOML11_HAS_STD_SOURCE_LOCATION
+
+// ----------------------------------------------------------------------------
+// (subset of) optional
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX17_VALUE
+#  if __has_include(<optional>)
+#    include <optional>
+#  endif // has_include(optional)
+#endif // C++17
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX17_VALUE
+#  if defined(__cpp_lib_optional)
+#    if __cpp_lib_optional >= 201606L
+#      define TOML11_HAS_STD_OPTIONAL 1
+#    endif
+#  endif
+#endif
+
+#if defined(TOML11_HAS_STD_OPTIONAL)
+
+namespace toml
+{
+namespace cxx
+{
+using std::optional;
+
+inline std::nullopt_t make_nullopt() {return std::nullopt;}
+
+template<typename charT, typename traitsT>
+std::basic_ostream<charT, traitsT>&
+operator<<(std::basic_ostream<charT, traitsT>& os, const std::nullopt_t&)
+{
+    os << "nullopt";
+    return os;
+}
+
+} // cxx
+} // toml
+
+#else // TOML11_HAS_STD_OPTIONAL
+
+namespace toml
+{
+namespace cxx
+{
+
+struct nullopt_t{};
+inline nullopt_t make_nullopt() {return nullopt_t{};}
+
+inline bool operator==(const nullopt_t&, const nullopt_t&) noexcept {return true;}
+inline bool operator!=(const nullopt_t&, const nullopt_t&) noexcept {return false;}
+inline bool operator< (const nullopt_t&, const nullopt_t&) noexcept {return false;}
+inline bool operator<=(const nullopt_t&, const nullopt_t&) noexcept {return true;}
+inline bool operator> (const nullopt_t&, const nullopt_t&) noexcept {return false;}
+inline bool operator>=(const nullopt_t&, const nullopt_t&) noexcept {return true;}
+
+template<typename charT, typename traitsT>
+std::basic_ostream<charT, traitsT>&
+operator<<(std::basic_ostream<charT, traitsT>& os, const nullopt_t&)
+{
+    os << "nullopt";
+    return os;
+}
+
+template<typename T>
+class optional
+{
+  public:
+
+    using value_type = T;
+
+  public:
+
+    optional() noexcept : has_value_(false), null_('\0') {}
+    optional(nullopt_t) noexcept : has_value_(false), null_('\0') {}
+
+    optional(const T& x): has_value_(true), value_(x) {}
+    optional(T&& x): has_value_(true), value_(std::move(x)) {}
+
+    template<typename U, enable_if_t<std::is_constructible<T, U>::value, std::nullptr_t> = nullptr>
+    explicit optional(U&& x): has_value_(true), value_(std::forward<U>(x)) {}
+
+    optional(const optional& rhs): has_value_(rhs.has_value_)
+    {
+        if(rhs.has_value_)
+        {
+            this->assigner(rhs.value_);
+        }
+    }
+    optional(optional&& rhs): has_value_(rhs.has_value_)
+    {
+        if(this->has_value_)
+        {
+            this->assigner(std::move(rhs.value_));
+        }
+    }
+
+    optional& operator=(const optional& rhs)
+    {
+        if(this == std::addressof(rhs)) {return *this;}
+
+        this->cleanup();
+        this->has_value_ = rhs.has_value_;
+        if(this->has_value_)
+        {
+            this->assigner(rhs.value_);
+        }
+        return *this;
+    }
+    optional& operator=(optional&& rhs)
+    {
+        if(this == std::addressof(rhs)) {return *this;}
+
+        this->cleanup();
+        this->has_value_ = rhs.has_value_;
+        if(this->has_value_)
+        {
+            this->assigner(std::move(rhs.value_));
+        }
+        return *this;
+    }
+
+    template<typename U, enable_if_t<conjunction<
+        negation<std::is_same<T, U>>, std::is_constructible<T, U>
+        >::value, std::nullptr_t> = nullptr>
+    explicit optional(const optional<U>& rhs): has_value_(rhs.has_value_), null_('\0')
+    {
+        if(rhs.has_value_)
+        {
+            this->assigner(rhs.value_);
+        }
+    }
+    template<typename U, enable_if_t<conjunction<
+        negation<std::is_same<T, U>>, std::is_constructible<T, U>
+        >::value, std::nullptr_t> = nullptr>
+    explicit optional(optional<U>&& rhs): has_value_(rhs.has_value_), null_('\0')
+    {
+        if(this->has_value_)
+        {
+            this->assigner(std::move(rhs.value_));
+        }
+    }
+
+    template<typename U, enable_if_t<conjunction<
+        negation<std::is_same<T, U>>, std::is_constructible<T, U>
+        >::value, std::nullptr_t> = nullptr>
+    optional& operator=(const optional<U>& rhs)
+    {
+        if(this == std::addressof(rhs)) {return *this;}
+
+        this->cleanup();
+        this->has_value_ = rhs.has_value_;
+        if(this->has_value_)
+        {
+            this->assigner(rhs.value_);
+        }
+        return *this;
+    }
+
+    template<typename U, enable_if_t<conjunction<
+        negation<std::is_same<T, U>>, std::is_constructible<T, U>
+        >::value, std::nullptr_t> = nullptr>
+    optional& operator=(optional<U>&& rhs)
+    {
+        if(this == std::addressof(rhs)) {return *this;}
+
+        this->cleanup();
+        this->has_value_ = rhs.has_value_;
+        if(this->has_value_)
+        {
+            this->assigner(std::move(rhs.value_));
+        }
+        return *this;
+    }
+    ~optional() noexcept
+    {
+        this->cleanup();
+    }
+
+    explicit operator bool() const noexcept
+    {
+        return has_value_;
+    }
+
+    bool has_value() const noexcept {return has_value_;}
+
+    value_type const& value(source_location loc = source_location::current()) const
+    {
+        if( ! this->has_value_)
+        {
+            throw std::runtime_error("optional::value(): bad_unwrap" + to_string(loc));
+        }
+        return this->value_;
+    }
+    value_type& value(source_location loc = source_location::current())
+    {
+        if( ! this->has_value_)
+        {
+            throw std::runtime_error("optional::value(): bad_unwrap" + to_string(loc));
+        }
+        return this->value_;
+    }
+
+    value_type const& value_or(const value_type& opt) const
+    {
+        if(this->has_value_) {return this->value_;} else {return opt;}
+    }
+    value_type& value_or(value_type& opt)
+    {
+        if(this->has_value_) {return this->value_;} else {return opt;}
+    }
+
+  private:
+
+    void cleanup() noexcept
+    {
+        if(this->has_value_)
+        {
+            value_.~T();
+        }
+    }
+
+    template<typename U>
+    void assigner(U&& x)
+    {
+        const auto tmp = ::new(std::addressof(this->value_)) value_type(std::forward<U>(x));
+        assert(tmp == std::addressof(this->value_));
+        (void)tmp;
+    }
+
+  private:
+
+    bool has_value_;
+    union
+    {
+        char null_;
+        T    value_;
+    };
+};
+} // cxx
+} // toml
+#endif // TOML11_HAS_STD_OPTIONAL
+
+#endif // TOML11_COMPAT_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/context.hpp b/src/frontend/qt_sdl/toml/toml11/context.hpp
new file mode 100644
index 00000000..cda038fb
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/context.hpp
@@ -0,0 +1,68 @@
+#ifndef TOML11_CONTEXT_HPP
+#define TOML11_CONTEXT_HPP
+
+#include "error_info.hpp"
+#include "spec.hpp"
+
+#include <vector>
+
+namespace toml
+{
+namespace detail
+{
+
+template<typename TypeConfig>
+class context
+{
+  public:
+
+    explicit context(const spec& toml_spec)
+        : toml_spec_(toml_spec), errors_{}
+    {}
+
+    bool has_error() const noexcept {return !errors_.empty();}
+
+    std::vector<error_info> const& errors() const noexcept {return errors_;}
+
+    semantic_version&       toml_version()       noexcept {return toml_spec_.version;}
+    semantic_version const& toml_version() const noexcept {return toml_spec_.version;}
+
+    spec&       toml_spec()       noexcept {return toml_spec_;}
+    spec const& toml_spec() const noexcept {return toml_spec_;}
+
+    void report_error(error_info err)
+    {
+        this->errors_.push_back(std::move(err));
+    }
+
+    error_info pop_last_error()
+    {
+        assert( ! errors_.empty());
+        auto e = std::move(errors_.back());
+        errors_.pop_back();
+        return e;
+    }
+
+  private:
+
+    spec toml_spec_;
+    std::vector<error_info> errors_;
+};
+
+} // detail
+} // toml
+
+#if defined(TOML11_COMPILE_SOURCES)
+namespace toml
+{
+struct type_config;
+struct ordered_type_config;
+namespace detail
+{
+extern template class context<::toml::type_config>;
+extern template class context<::toml::ordered_type_config>;
+} // detail
+} // toml
+#endif // TOML11_COMPILE_SOURCES
+
+#endif // TOML11_CONTEXT_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/macros.hpp b/src/frontend/qt_sdl/toml/toml11/conversion.hpp
similarity index 77%
rename from src/frontend/qt_sdl/toml/toml/macros.hpp
rename to src/frontend/qt_sdl/toml/toml11/conversion.hpp
index e8f91aec..819a7658 100644
--- a/src/frontend/qt_sdl/toml/toml/macros.hpp
+++ b/src/frontend/qt_sdl/toml/toml11/conversion.hpp
@@ -1,5 +1,105 @@
-#ifndef TOML11_MACROS_HPP
-#define TOML11_MACROS_HPP
+#ifndef TOML11_CONVERSION_HPP
+#define TOML11_CONVERSION_HPP
+
+#include "find.hpp"
+#include "from.hpp" // IWYU pragma: keep
+#include "into.hpp" // IWYU pragma: keep
+
+#if defined(TOML11_HAS_OPTIONAL)
+
+#include <optional>
+
+namespace toml
+{
+namespace detail
+{
+
+template<typename T>
+inline constexpr bool is_optional_v = false;
+
+template<typename T>
+inline constexpr bool is_optional_v<std::optional<T>> = true;
+
+template<typename T, typename TC>
+void find_member_variable_from_value(T& obj, const basic_value<TC>& v, const char* var_name)
+{
+    if constexpr(is_optional_v<T>)
+    {
+        if(v.contains(var_name))
+        {
+            obj = toml::find<typename T::value_type>(v, var_name);
+        }
+        else
+        {
+            obj = std::nullopt;
+        }
+    }
+    else
+    {
+        obj = toml::find<T>(v, var_name);
+    }
+}
+
+template<typename T, typename TC>
+void assign_member_variable_to_value(const T& obj, basic_value<TC>& v, const char* var_name)
+{
+    if constexpr(is_optional_v<T>)
+    {
+        if(obj.has_value())
+        {
+            v[var_name] = obj.value();
+        }
+    }
+    else
+    {
+        v[var_name] = obj;
+    }
+}
+
+} // detail
+} // toml
+
+#else
+
+namespace toml
+{
+namespace detail
+{
+
+template<typename T, typename TC>
+void find_member_variable_from_value(T& obj, const basic_value<TC>& v, const char* var_name)
+{
+    obj = toml::find<T>(v, var_name);
+}
+
+template<typename T, typename TC>
+void assign_member_variable_to_value(const T& obj, basic_value<TC>& v, const char* var_name)
+{
+    v[var_name] = obj;
+}
+
+} // detail
+} // toml
+
+#endif // optional
+
+// use it in the following way.
+// ```cpp
+// namespace foo
+// {
+// struct Foo
+// {
+//     std::string s;
+//     double      d;
+//     int         i;
+// };
+// } // foo
+//
+// TOML11_DEFINE_CONVERSION_NON_INTRUSIVE(foo::Foo, s, d, i)
+// ```
+//
+// And then you can use `toml::get<foo::Foo>(v)` and `toml::find<foo::Foo>(file, "foo");`
+//
 
 #define TOML11_STRINGIZE_AUX(x) #x
 #define TOML11_STRINGIZE(x)     TOML11_STRINGIZE_AUX(x)
@@ -65,39 +165,20 @@
 #define TOML11_FOR_EACH_VA_ARGS(FUNCTOR, ...)\
     TOML11_CONCATENATE(TOML11_FOR_EACH_VA_ARGS_AUX_, TOML11_ARGS_SIZE(__VA_ARGS__))(FUNCTOR, __VA_ARGS__)
 
-// ----------------------------------------------------------------------------
-// TOML11_DEFINE_CONVERSION_NON_INTRUSIVE
 
-// use it in the following way.
-// ```cpp
-// namespace foo
-// {
-// struct Foo
-// {
-//     std::string s;
-//     double      d;
-//     int         i;
-// };
-// } // foo
-//
-// TOML11_DEFINE_CONVERSION_NON_INTRUSIVE(foo::Foo, s, d, i)
-// ```
-// And then you can use `toml::find<foo::Foo>(file, "foo");`
-//
 #define TOML11_FIND_MEMBER_VARIABLE_FROM_VALUE(VAR_NAME)\
-    obj.VAR_NAME = toml::find<decltype(obj.VAR_NAME)>(v, TOML11_STRINGIZE(VAR_NAME));
+    toml::detail::find_member_variable_from_value(obj.VAR_NAME, v, TOML11_STRINGIZE(VAR_NAME));
 
 #define TOML11_ASSIGN_MEMBER_VARIABLE_TO_VALUE(VAR_NAME)\
-    v[TOML11_STRINGIZE(VAR_NAME)] = obj.VAR_NAME;
+    toml::detail::assign_member_variable_to_value(obj.VAR_NAME, v, TOML11_STRINGIZE(VAR_NAME));
 
 #define TOML11_DEFINE_CONVERSION_NON_INTRUSIVE(NAME, ...)\
     namespace toml {                                                                     \
     template<>                                                                           \
     struct from<NAME>                                                                    \
     {                                                                                    \
-        template<typename C, template<typename ...> class T,                             \
-                 template<typename ...> class A>                                         \
-        static NAME from_toml(const basic_value<C, T, A>& v)                             \
+        template<typename TC>                                                            \
+        static NAME from_toml(const basic_value<TC>& v)                                  \
         {                                                                                \
             NAME obj;                                                                    \
             TOML11_FOR_EACH_VA_ARGS(TOML11_FIND_MEMBER_VARIABLE_FROM_VALUE, __VA_ARGS__) \
@@ -107,9 +188,10 @@
     template<>                                                                           \
     struct into<NAME>                                                                    \
     {                                                                                    \
-        static value into_toml(const NAME& obj)                                          \
+        template<typename TC>                                                            \
+        static basic_value<TC> into_toml(const NAME& obj)                                \
         {                                                                                \
-            ::toml::value v = ::toml::table{};                                           \
+            ::toml::basic_value<TC> v = typename ::toml::basic_value<TC>::table_type{};  \
             TOML11_FOR_EACH_VA_ARGS(TOML11_ASSIGN_MEMBER_VARIABLE_TO_VALUE, __VA_ARGS__) \
             return v;                                                                    \
         }                                                                                \
@@ -118,4 +200,4 @@
 
 #endif// TOML11_WITHOUT_DEFINE_NON_INTRUSIVE
 
-#endif// TOML11_MACROS_HPP
+#endif // TOML11_CONVERSION_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/datetime.hpp b/src/frontend/qt_sdl/toml/toml11/datetime.hpp
new file mode 100644
index 00000000..08411800
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/datetime.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_DATETIME_HPP
+#define TOML11_DATETIME_HPP
+
+#include "fwd/datetime_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/datetime_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_DATETIME_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/error_info.hpp b/src/frontend/qt_sdl/toml/toml11/error_info.hpp
new file mode 100644
index 00000000..4575f7ac
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/error_info.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_ERROR_INFO_HPP
+#define TOML11_ERROR_INFO_HPP
+
+#include "fwd/error_info_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/error_info_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_ERROR_INFO_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/exception.hpp b/src/frontend/qt_sdl/toml/toml11/exception.hpp
new file mode 100644
index 00000000..7149eb3c
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/exception.hpp
@@ -0,0 +1,17 @@
+#ifndef TOML11_EXCEPTION_HPP
+#define TOML11_EXCEPTION_HPP
+
+#include <exception>
+
+namespace toml
+{
+
+struct exception : public std::exception
+{
+  public:
+    virtual ~exception() noexcept override = default;
+    virtual const char* what() const noexcept override {return "";}
+};
+
+} // toml
+#endif // TOMl11_EXCEPTION_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/find.hpp b/src/frontend/qt_sdl/toml/toml11/find.hpp
new file mode 100644
index 00000000..560db2a7
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/find.hpp
@@ -0,0 +1,377 @@
+#ifndef TOML11_FIND_HPP
+#define TOML11_FIND_HPP
+
+#include <algorithm>
+
+#include "get.hpp"
+#include "value.hpp"
+
+#if defined(TOML11_HAS_STRING_VIEW)
+#include <string_view>
+#endif
+
+namespace toml
+{
+
+// ----------------------------------------------------------------------------
+// find<T>(value, key);
+
+template<typename T, typename TC>
+decltype(::toml::get<T>(std::declval<basic_value<TC> const&>()))
+find(const basic_value<TC>& v, const typename basic_value<TC>::key_type& ky)
+{
+    return ::toml::get<T>(v.at(ky));
+}
+
+template<typename T, typename TC>
+decltype(::toml::get<T>(std::declval<basic_value<TC>&>()))
+find(basic_value<TC>& v, const typename basic_value<TC>::key_type& ky)
+{
+    return ::toml::get<T>(v.at(ky));
+}
+
+template<typename T, typename TC>
+decltype(::toml::get<T>(std::declval<basic_value<TC>&&>()))
+find(basic_value<TC>&& v, const typename basic_value<TC>::key_type& ky)
+{
+    return ::toml::get<T>(std::move(v.at(ky)));
+}
+
+// ----------------------------------------------------------------------------
+// find<T>(value, idx)
+
+template<typename T, typename TC>
+decltype(::toml::get<T>(std::declval<basic_value<TC> const&>()))
+find(const basic_value<TC>& v, const std::size_t idx)
+{
+    return ::toml::get<T>(v.at(idx));
+}
+template<typename T, typename TC>
+decltype(::toml::get<T>(std::declval<basic_value<TC>&>()))
+find(basic_value<TC>& v, const std::size_t idx)
+{
+    return ::toml::get<T>(v.at(idx));
+}
+template<typename T, typename TC>
+decltype(::toml::get<T>(std::declval<basic_value<TC>&&>()))
+find(basic_value<TC>&& v, const std::size_t idx)
+{
+    return ::toml::get<T>(std::move(v.at(idx)));
+}
+
+// ----------------------------------------------------------------------------
+// find(value, key/idx), w/o conversion
+
+template<typename TC>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>&
+find(basic_value<TC>& v, const typename basic_value<TC>::key_type& ky)
+{
+    return v.at(ky);
+}
+template<typename TC>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>> const&
+find(basic_value<TC> const& v, const typename basic_value<TC>::key_type& ky)
+{
+    return v.at(ky);
+}
+template<typename TC>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>
+find(basic_value<TC>&& v, const typename basic_value<TC>::key_type& ky)
+{
+    return basic_value<TC>(std::move(v.at(ky)));
+}
+
+template<typename TC>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>&
+find(basic_value<TC>& v, const std::size_t idx)
+{
+    return v.at(idx);
+}
+template<typename TC>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>> const&
+find(basic_value<TC> const& v, const std::size_t idx)
+{
+    return v.at(idx);
+}
+template<typename TC>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>
+find(basic_value<TC>&& v, const std::size_t idx)
+{
+    return basic_value<TC>(std::move(v.at(idx)));
+}
+
+// --------------------------------------------------------------------------
+// toml::find(toml::value, toml::key, Ts&& ... keys)
+
+namespace detail
+{
+
+// It suppresses warnings by -Wsign-conversion when we pass integer literal
+// to toml::find. integer literal `0` is deduced as an int, and will be
+// converted to std::size_t. This causes sign-conversion.
+
+template<typename TC>
+std::size_t key_cast(const std::size_t& v) noexcept
+{
+    return v;
+}
+template<typename TC, typename T>
+cxx::enable_if_t<std::is_integral<cxx::remove_cvref_t<T>>::value, std::size_t>
+key_cast(const T& v) noexcept
+{
+    return static_cast<std::size_t>(v);
+}
+
+// for string-like (string, string literal, string_view)
+
+template<typename TC>
+typename basic_value<TC>::key_type const&
+key_cast(const typename basic_value<TC>::key_type& v) noexcept
+{
+    return v;
+}
+template<typename TC>
+typename basic_value<TC>::key_type
+key_cast(const typename basic_value<TC>::key_type::value_type* v)
+{
+    return typename basic_value<TC>::key_type(v);
+}
+#if defined(TOML11_HAS_STRING_VIEW)
+template<typename TC>
+typename basic_value<TC>::key_type
+key_cast(const std::string_view v)
+{
+    return typename basic_value<TC>::key_type(v);
+}
+#endif // string_view
+
+} // detail
+
+// ----------------------------------------------------------------------------
+// find(v, keys...)
+
+template<typename TC, typename K1, typename K2, typename ... Ks>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>> const&
+find(const basic_value<TC>& v, const K1& k1, const K2& k2, const Ks& ... ks)
+{
+    return find(v.at(detail::key_cast<TC>(k1)), detail::key_cast<TC>(k2), ks...);
+}
+template<typename TC, typename K1, typename K2, typename ... Ks>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>&
+find(basic_value<TC>& v, const K1& k1, const K2& k2, const Ks& ... ks)
+{
+    return find(v.at(detail::key_cast<TC>(k1)), detail::key_cast<TC>(k2), ks...);
+}
+template<typename TC, typename K1, typename K2, typename ... Ks>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>
+find(basic_value<TC>&& v, const K1& k1, const K2& k2, const Ks& ... ks)
+{
+    return find(std::move(v.at(detail::key_cast<TC>(k1))), detail::key_cast<TC>(k2), ks...);
+}
+
+// ----------------------------------------------------------------------------
+// find<T>(v, keys...)
+
+template<typename T, typename TC, typename K1, typename K2, typename ... Ks>
+decltype(::toml::get<T>(std::declval<const basic_value<TC>&>()))
+find(const basic_value<TC>& v, const K1& k1, const K2& k2, const Ks& ... ks)
+{
+    return find<T>(v.at(detail::key_cast<TC>(k1)), detail::key_cast<TC>(k2), ks...);
+}
+template<typename T, typename TC, typename K1, typename K2, typename ... Ks>
+decltype(::toml::get<T>(std::declval<basic_value<TC>&>()))
+find(basic_value<TC>& v, const K1& k1, const K2& k2, const Ks& ... ks)
+{
+    return find<T>(v.at(detail::key_cast<TC>(k1)), detail::key_cast<TC>(k2), ks...);
+}
+template<typename T, typename TC, typename K1, typename K2, typename ... Ks>
+decltype(::toml::get<T>(std::declval<basic_value<TC>&&>()))
+find(basic_value<TC>&& v, const K1& k1, const K2& k2, const Ks& ... ks)
+{
+    return find<T>(std::move(v.at(detail::key_cast<TC>(k1))), detail::key_cast<TC>(k2), ks...);
+}
+
+// ===========================================================================
+// find_or<T>(value, key, fallback)
+
+// ---------------------------------------------------------------------------
+// find_or(v, key, other_v)
+
+template<typename TC, typename K>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>&
+find_or(basic_value<TC>& v, const K& k, basic_value<TC>& opt) noexcept
+{
+    try
+    {
+        return ::toml::find(v, detail::key_cast<TC>(k));
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+template<typename TC, typename K>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>> const&
+find_or(const basic_value<TC>& v, const K& k, const basic_value<TC>& opt) noexcept
+{
+    try
+    {
+        return ::toml::find(v, detail::key_cast<TC>(k));
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+template<typename TC, typename K>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>
+find_or(basic_value<TC>&& v, const K& k, basic_value<TC>&& opt) noexcept
+{
+    try
+    {
+        return ::toml::find(v, detail::key_cast<TC>(k));
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+
+// ---------------------------------------------------------------------------
+// toml types (return type can be a reference)
+
+template<typename T, typename TC, typename K>
+cxx::enable_if_t<detail::is_exact_toml_type<T, basic_value<TC>>::value,
+    cxx::remove_cvref_t<T> const&>
+find_or(const basic_value<TC>& v, const K& k, const T& opt)
+{
+    try
+    {
+        return ::toml::get<T>(v.at(detail::key_cast<TC>(k)));
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+
+template<typename T, typename TC, typename K>
+cxx::enable_if_t<cxx::conjunction<
+        cxx::negation<std::is_const<T>>,
+        detail::is_exact_toml_type<T, basic_value<TC>>
+    >::value, cxx::remove_cvref_t<T>&>
+find_or(basic_value<TC>& v, const K& k, T& opt)
+{
+    try
+    {
+        return ::toml::get<T>(v.at(detail::key_cast<TC>(k)));
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+
+template<typename T, typename TC, typename K>
+cxx::enable_if_t<detail::is_exact_toml_type<T, basic_value<TC>>::value,
+    cxx::remove_cvref_t<T>>
+find_or(basic_value<TC>&& v, const K& k, T opt)
+{
+    try
+    {
+        return ::toml::get<T>(std::move(v.at(detail::key_cast<TC>(k))));
+    }
+    catch(...)
+    {
+        return T(std::move(opt));
+    }
+}
+
+// ---------------------------------------------------------------------------
+// string literal (deduced as std::string)
+
+// XXX to avoid confusion when T is explicitly specified in find_or<T>(),
+//     we restrict the string type as std::string.
+template<typename TC, typename K>
+cxx::enable_if_t<detail::is_type_config<TC>::value, std::string>
+find_or(const basic_value<TC>& v, const K& k, const char* opt)
+{
+    try
+    {
+        return ::toml::get<std::string>(v.at(detail::key_cast<TC>(k)));
+    }
+    catch(...)
+    {
+        return std::string(opt);
+    }
+}
+
+// ---------------------------------------------------------------------------
+// other types (requires type conversion and return type cannot be a reference)
+
+template<typename T, typename TC, typename K>
+cxx::enable_if_t<cxx::conjunction<
+        cxx::negation<detail::is_basic_value<cxx::remove_cvref_t<T>>>,
+        detail::is_not_toml_type<cxx::remove_cvref_t<T>, basic_value<TC>>,
+        cxx::negation<std::is_same<cxx::remove_cvref_t<T>,
+            const typename basic_value<TC>::string_type::value_type*>>
+    >::value, cxx::remove_cvref_t<T>>
+find_or(const basic_value<TC>& v, const K& ky, T opt)
+{
+    try
+    {
+        return ::toml::get<cxx::remove_cvref_t<T>>(v.at(detail::key_cast<TC>(ky)));
+    }
+    catch(...)
+    {
+        return cxx::remove_cvref_t<T>(std::move(opt));
+    }
+}
+
+// ----------------------------------------------------------------------------
+// recursive
+
+namespace detail
+{
+
+template<typename ...Ts>
+auto last_one(Ts&&... args)
+ -> decltype(std::get<sizeof...(Ts)-1>(std::forward_as_tuple(std::forward<Ts>(args)...)))
+{
+    return std::get<sizeof...(Ts)-1>(std::forward_as_tuple(std::forward<Ts>(args)...));
+}
+
+} // detail
+
+template<typename Value, typename K1, typename K2, typename K3, typename ... Ks>
+auto find_or(Value&& v, const K1& k1, const K2& k2, K3&& k3, Ks&& ... keys) noexcept
+    -> cxx::enable_if_t<
+        detail::is_basic_value<cxx::remove_cvref_t<Value>>::value,
+        decltype(find_or(v, k2, std::forward<K3>(k3), std::forward<Ks>(keys)...))
+    >
+{
+    try
+    {
+        return find_or(v.at(k1), k2, std::forward<K3>(k3), std::forward<Ks>(keys)...);
+    }
+    catch(...)
+    {
+        return detail::last_one(k3, keys...);
+    }
+}
+
+template<typename T, typename TC, typename K1, typename K2, typename K3, typename ... Ks>
+T find_or(const basic_value<TC>& v, const K1& k1, const K2& k2, const K3& k3, const Ks& ... keys) noexcept
+{
+    try
+    {
+        return find_or<T>(v.at(k1), k2, k3, keys...);
+    }
+    catch(...)
+    {
+        return static_cast<T>(detail::last_one(k3, keys...));
+    }
+}
+
+} // toml
+#endif // TOML11_FIND_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/format.hpp b/src/frontend/qt_sdl/toml/toml11/format.hpp
new file mode 100644
index 00000000..6662220d
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/format.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_FORMAT_HPP
+#define TOML11_FORMAT_HPP
+
+#include "fwd/format_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/format_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif// TOML11_FORMAT_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/from.hpp b/src/frontend/qt_sdl/toml/toml11/from.hpp
similarity index 78%
rename from src/frontend/qt_sdl/toml/toml/from.hpp
rename to src/frontend/qt_sdl/toml/toml11/from.hpp
index 10815caf..d2e0e132 100644
--- a/src/frontend/qt_sdl/toml/toml/from.hpp
+++ b/src/frontend/qt_sdl/toml/toml11/from.hpp
@@ -1,5 +1,3 @@
-//     Copyright Toru Niina 2019.
-// Distributed under the MIT License.
 #ifndef TOML11_FROM_HPP
 #define TOML11_FROM_HPP
 
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/color_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/color_fwd.hpp
new file mode 100644
index 00000000..ed711c0d
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/color_fwd.hpp
@@ -0,0 +1,88 @@
+#ifndef TOML11_COLOR_FWD_HPP
+#define TOML11_COLOR_FWD_HPP
+
+#include <iosfwd>
+
+#ifdef TOML11_COLORIZE_ERROR_MESSAGE
+#define TOML11_ERROR_MESSAGE_COLORIZED true
+#else
+#define TOML11_ERROR_MESSAGE_COLORIZED false
+#endif
+
+#ifdef TOML11_USE_THREAD_LOCAL_COLORIZATION
+#define TOML11_THREAD_LOCAL_COLORIZATION thread_local
+#else
+#define TOML11_THREAD_LOCAL_COLORIZATION
+#endif
+
+namespace toml
+{
+namespace color
+{
+// put ANSI escape sequence to ostream
+inline namespace ansi
+{
+namespace detail
+{
+
+// Control color mode globally
+class color_mode
+{
+  public:
+
+    void enable() noexcept
+    {
+        should_color_ = true;
+    }
+    void disable() noexcept
+    {
+        should_color_ = false;
+    }
+    bool should_color() const noexcept
+    {
+        return should_color_;
+    }
+
+  private:
+
+    bool should_color_ = TOML11_ERROR_MESSAGE_COLORIZED;
+};
+
+inline color_mode& color_status() noexcept
+{
+    static TOML11_THREAD_LOCAL_COLORIZATION color_mode status;
+    return status;
+}
+
+} // detail
+
+std::ostream& reset  (std::ostream& os);
+std::ostream& bold   (std::ostream& os);
+std::ostream& grey   (std::ostream& os);
+std::ostream& gray   (std::ostream& os);
+std::ostream& red    (std::ostream& os);
+std::ostream& green  (std::ostream& os);
+std::ostream& yellow (std::ostream& os);
+std::ostream& blue   (std::ostream& os);
+std::ostream& magenta(std::ostream& os);
+std::ostream& cyan   (std::ostream& os);
+std::ostream& white  (std::ostream& os);
+
+} // ansi
+
+inline void enable()
+{
+    return detail::color_status().enable();
+}
+inline void disable()
+{
+    return detail::color_status().disable();
+}
+inline bool should_color()
+{
+    return detail::color_status().should_color();
+}
+
+} // color
+} // toml
+#endif // TOML11_COLOR_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/comments.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/comments_fwd.hpp
similarity index 86%
rename from src/frontend/qt_sdl/toml/toml/comments.hpp
rename to src/frontend/qt_sdl/toml/toml11/fwd/comments_fwd.hpp
index ec250411..bbc9926b 100644
--- a/src/frontend/qt_sdl/toml/toml/comments.hpp
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/comments_fwd.hpp
@@ -1,7 +1,10 @@
-//     Copyright Toru Niina 2019.
-// Distributed under the MIT License.
-#ifndef TOML11_COMMENTS_HPP
-#define TOML11_COMMENTS_HPP
+#ifndef TOML11_COMMENTS_FWD_HPP
+#define TOML11_COMMENTS_FWD_HPP
+
+// to use __has_builtin
+#include "../version.hpp" // IWYU pragma: keep
+
+#include <exception>
 #include <initializer_list>
 #include <iterator>
 #include <stdexcept>
@@ -9,12 +12,7 @@
 #include <type_traits>
 #include <utility>
 #include <vector>
-
-#ifdef TOML11_PRESERVE_COMMENTS_BY_DEFAULT
-#  define TOML11_DEFAULT_COMMENT_STRATEGY ::toml::preserve_comments
-#else
-#  define TOML11_DEFAULT_COMMENT_STRATEGY ::toml::discard_comments
-#endif
+#include <ostream>
 
 // This file provides mainly two classes, `preserve_comments` and `discard_comments`.
 // Those two are a container that have the same interface as `std::vector<std::string>`
@@ -25,16 +23,11 @@
 // error whenever you access to the element.
 namespace toml
 {
-struct discard_comments; // forward decl
+class discard_comments; // forward decl
 
-// use it in the following way
-//
-// const toml::basic_value<toml::preserve_comments> data =
-//     toml::parse<toml::preserve_comments>("example.toml");
-//
-// the interface is almost the same as std::vector<std::string>.
-struct preserve_comments
+class preserve_comments
 {
+  public:
     // `container_type` is not provided in discard_comments.
     // do not use this inner-type in a generic code.
     using container_type         = std::vector<std::string>;
@@ -51,6 +44,8 @@ struct preserve_comments
     using reverse_iterator       = container_type::reverse_iterator;
     using const_reverse_iterator = container_type::const_reverse_iterator;
 
+  public:
+
     preserve_comments()  = default;
     ~preserve_comments() = default;
     preserve_comments(preserve_comments const&) = default;
@@ -90,11 +85,9 @@ struct preserve_comments
 
     // Related to the issue #97.
     //
-    // It is known that `std::vector::insert` and `std::vector::erase` in
-    // the standard library implementation included in GCC 4.8.5 takes
-    // `std::vector::iterator` instead of `std::vector::const_iterator`.
-    // Because of the const-correctness, we cannot convert a `const_iterator` to
-    // an `iterator`. It causes compilation error in GCC 4.8.5.
+    // `std::vector::insert` and `std::vector::erase` in the STL implementation
+    // included in GCC 4.8.5 takes `std::vector::iterator` instead of
+    // `std::vector::const_iterator`. It causes compilation error in GCC 4.8.5.
 #if defined(__GNUC__) && defined(__GNUC_MINOR__) && defined(__GNUC_PATCHLEVEL__) && !defined(__clang__)
 #  if (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) <= 40805
 #    define TOML11_WORKAROUND_GCC_4_8_X_STANDARD_LIBRARY_IMPLEMENTATION
@@ -233,39 +226,18 @@ struct preserve_comments
     container_type comments;
 };
 
-inline bool operator==(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments == rhs.comments;}
-inline bool operator!=(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments != rhs.comments;}
-inline bool operator< (const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments <  rhs.comments;}
-inline bool operator<=(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments <= rhs.comments;}
-inline bool operator> (const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments >  rhs.comments;}
-inline bool operator>=(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments >= rhs.comments;}
+bool operator==(const preserve_comments& lhs, const preserve_comments& rhs);
+bool operator!=(const preserve_comments& lhs, const preserve_comments& rhs);
+bool operator< (const preserve_comments& lhs, const preserve_comments& rhs);
+bool operator<=(const preserve_comments& lhs, const preserve_comments& rhs);
+bool operator> (const preserve_comments& lhs, const preserve_comments& rhs);
+bool operator>=(const preserve_comments& lhs, const preserve_comments& rhs);
 
-inline void swap(preserve_comments& lhs, preserve_comments& rhs)
-{
-    lhs.swap(rhs);
-    return;
-}
-inline void swap(preserve_comments& lhs, std::vector<std::string>& rhs)
-{
-    lhs.comments.swap(rhs);
-    return;
-}
-inline void swap(std::vector<std::string>& lhs, preserve_comments& rhs)
-{
-    lhs.swap(rhs.comments);
-    return;
-}
+void swap(preserve_comments& lhs, preserve_comments& rhs);
+void swap(preserve_comments& lhs, std::vector<std::string>& rhs);
+void swap(std::vector<std::string>& lhs, preserve_comments& rhs);
 
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, const preserve_comments& com)
-{
-    for(const auto& c : com)
-    {
-        os << '#' << c << '\n';
-    }
-    return os;
-}
+std::ostream& operator<<(std::ostream& os, const preserve_comments& com);
 
 namespace detail
 {
@@ -349,8 +321,9 @@ operator+(const empty_iterator<T, C>& lhs, typename empty_iterator<T, C>::differ
 // efficiency, this is chosen as a default.
 //
 // To reduce the memory footprint, later we can try empty base optimization (EBO).
-struct discard_comments
+class discard_comments
 {
+  public:
     using size_type              = std::size_t;
     using difference_type        = std::ptrdiff_t;
     using value_type             = std::string;
@@ -363,6 +336,7 @@ struct discard_comments
     using reverse_iterator       = detail::empty_iterator<std::string, false>;
     using const_reverse_iterator = detail::empty_iterator<std::string, true>;
 
+  public:
     discard_comments() = default;
     ~discard_comments() = default;
     discard_comments(discard_comments const&) = default;
@@ -425,14 +399,14 @@ struct discard_comments
     // empty, so accessing through operator[], front/back, data causes address
     // error.
 
-    reference       operator[](const size_type)       noexcept {return *data();}
-    const_reference operator[](const size_type) const noexcept {return *data();}
+    reference       operator[](const size_type)       noexcept {never_call("toml::discard_comment::operator[]");}
+    const_reference operator[](const size_type) const noexcept {never_call("toml::discard_comment::operator[]");}
     reference       at(const size_type)       {throw std::out_of_range("toml::discard_comment is always empty.");}
     const_reference at(const size_type) const {throw std::out_of_range("toml::discard_comment is always empty.");}
-    reference       front()       noexcept {return *data();}
-    const_reference front() const noexcept {return *data();}
-    reference       back()        noexcept {return *data();}
-    const_reference back()  const noexcept {return *data();}
+    reference       front()       noexcept {never_call("toml::discard_comment::front");}
+    const_reference front() const noexcept {never_call("toml::discard_comment::front");}
+    reference       back()        noexcept {never_call("toml::discard_comment::back");}
+    const_reference back()  const noexcept {never_call("toml::discard_comment::back");}
 
     pointer         data()        noexcept {return nullptr;}
     const_pointer   data()  const noexcept {return nullptr;}
@@ -450,6 +424,16 @@ struct discard_comments
     const_reverse_iterator rend()    const noexcept {return const_iterator{};}
     const_reverse_iterator crbegin() const noexcept {return const_iterator{};}
     const_reverse_iterator crend()   const noexcept {return const_iterator{};}
+
+  private:
+
+    [[noreturn]] static void never_call(const char *const this_function)
+    {
+#if __has_builtin(__builtin_unreachable)
+        __builtin_unreachable();
+#endif
+        throw std::logic_error{this_function};
+    }
 };
 
 inline bool operator==(const discard_comments&, const discard_comments&) noexcept {return true;}
@@ -461,12 +445,7 @@ inline bool operator>=(const discard_comments&, const discard_comments&) noexcep
 
 inline void swap(const discard_comments&, const discard_comments&) noexcept {return;}
 
-template<typename charT, typename traits>
-std::basic_ostream<charT, traits>&
-operator<<(std::basic_ostream<charT, traits>& os, const discard_comments&)
-{
-    return os;
-}
+inline std::ostream& operator<<(std::ostream& os, const discard_comments&) {return os;}
 
 } // toml11
-#endif// TOML11_COMMENTS_HPP
+#endif // TOML11_COMMENTS_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/datetime_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/datetime_fwd.hpp
new file mode 100644
index 00000000..44616a17
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/datetime_fwd.hpp
@@ -0,0 +1,261 @@
+#ifndef TOML11_DATETIME_FWD_HPP
+#define TOML11_DATETIME_FWD_HPP
+
+#include <chrono>
+#include <iosfwd>
+#include <string>
+
+#include <cstdint>
+#include <cstdlib>
+#include <ctime>
+
+namespace toml
+{
+
+enum class month_t : std::uint8_t
+{
+    Jan =  0,
+    Feb =  1,
+    Mar =  2,
+    Apr =  3,
+    May =  4,
+    Jun =  5,
+    Jul =  6,
+    Aug =  7,
+    Sep =  8,
+    Oct =  9,
+    Nov = 10,
+    Dec = 11
+};
+
+// ----------------------------------------------------------------------------
+
+struct local_date
+{
+    std::int16_t year{0};   // A.D. (like, 2018)
+    std::uint8_t month{0};  // [0, 11]
+    std::uint8_t day{0};    // [1, 31]
+
+    local_date(int y, month_t m, int d)
+        : year {static_cast<std::int16_t>(y)},
+          month{static_cast<std::uint8_t>(m)},
+          day  {static_cast<std::uint8_t>(d)}
+    {}
+
+    explicit local_date(const std::tm& t)
+        : year {static_cast<std::int16_t>(t.tm_year + 1900)},
+          month{static_cast<std::uint8_t>(t.tm_mon)},
+          day  {static_cast<std::uint8_t>(t.tm_mday)}
+    {}
+
+    explicit local_date(const std::chrono::system_clock::time_point& tp);
+    explicit local_date(const std::time_t t);
+
+    operator std::chrono::system_clock::time_point() const;
+    operator std::time_t() const;
+
+    local_date() = default;
+    ~local_date() = default;
+    local_date(local_date const&) = default;
+    local_date(local_date&&)      = default;
+    local_date& operator=(local_date const&) = default;
+    local_date& operator=(local_date&&)      = default;
+};
+bool operator==(const local_date& lhs, const local_date& rhs);
+bool operator!=(const local_date& lhs, const local_date& rhs);
+bool operator< (const local_date& lhs, const local_date& rhs);
+bool operator<=(const local_date& lhs, const local_date& rhs);
+bool operator> (const local_date& lhs, const local_date& rhs);
+bool operator>=(const local_date& lhs, const local_date& rhs);
+
+std::ostream& operator<<(std::ostream& os, const local_date& date);
+std::string to_string(const local_date& date);
+
+// -----------------------------------------------------------------------------
+
+struct local_time
+{
+    std::uint8_t  hour{0};        // [0, 23]
+    std::uint8_t  minute{0};      // [0, 59]
+    std::uint8_t  second{0};      // [0, 60]
+    std::uint16_t millisecond{0}; // [0, 999]
+    std::uint16_t microsecond{0}; // [0, 999]
+    std::uint16_t nanosecond{0};  // [0, 999]
+
+    local_time(int h, int m, int s,
+               int ms = 0, int us = 0, int ns = 0)
+        : hour  {static_cast<std::uint8_t>(h)},
+          minute{static_cast<std::uint8_t>(m)},
+          second{static_cast<std::uint8_t>(s)},
+          millisecond{static_cast<std::uint16_t>(ms)},
+          microsecond{static_cast<std::uint16_t>(us)},
+          nanosecond {static_cast<std::uint16_t>(ns)}
+    {}
+
+    explicit local_time(const std::tm& t)
+        : hour  {static_cast<std::uint8_t>(t.tm_hour)},
+          minute{static_cast<std::uint8_t>(t.tm_min )},
+          second{static_cast<std::uint8_t>(t.tm_sec )},
+          millisecond{0}, microsecond{0}, nanosecond{0}
+    {}
+
+    template<typename Rep, typename Period>
+    explicit local_time(const std::chrono::duration<Rep, Period>& t)
+    {
+        const auto h = std::chrono::duration_cast<std::chrono::hours>(t);
+        this->hour = static_cast<std::uint8_t>(h.count());
+        const auto t2 = t - h;
+        const auto m = std::chrono::duration_cast<std::chrono::minutes>(t2);
+        this->minute = static_cast<std::uint8_t>(m.count());
+        const auto t3 = t2 - m;
+        const auto s = std::chrono::duration_cast<std::chrono::seconds>(t3);
+        this->second = static_cast<std::uint8_t>(s.count());
+        const auto t4 = t3 - s;
+        const auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(t4);
+        this->millisecond = static_cast<std::uint16_t>(ms.count());
+        const auto t5 = t4 - ms;
+        const auto us = std::chrono::duration_cast<std::chrono::microseconds>(t5);
+        this->microsecond = static_cast<std::uint16_t>(us.count());
+        const auto t6 = t5 - us;
+        const auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(t6);
+        this->nanosecond = static_cast<std::uint16_t>(ns.count());
+    }
+
+    operator std::chrono::nanoseconds() const;
+
+    local_time() = default;
+    ~local_time() = default;
+    local_time(local_time const&) = default;
+    local_time(local_time&&)      = default;
+    local_time& operator=(local_time const&) = default;
+    local_time& operator=(local_time&&)      = default;
+};
+
+bool operator==(const local_time& lhs, const local_time& rhs);
+bool operator!=(const local_time& lhs, const local_time& rhs);
+bool operator< (const local_time& lhs, const local_time& rhs);
+bool operator<=(const local_time& lhs, const local_time& rhs);
+bool operator> (const local_time& lhs, const local_time& rhs);
+bool operator>=(const local_time& lhs, const local_time& rhs);
+
+std::ostream& operator<<(std::ostream& os, const local_time& time);
+std::string to_string(const local_time& time);
+
+// ----------------------------------------------------------------------------
+
+struct time_offset
+{
+    std::int8_t hour{0};   // [-12, 12]
+    std::int8_t minute{0}; // [-59, 59]
+
+    time_offset(int h, int m)
+        : hour  {static_cast<std::int8_t>(h)},
+          minute{static_cast<std::int8_t>(m)}
+    {}
+
+    operator std::chrono::minutes() const;
+
+    time_offset() = default;
+    ~time_offset() = default;
+    time_offset(time_offset const&) = default;
+    time_offset(time_offset&&)      = default;
+    time_offset& operator=(time_offset const&) = default;
+    time_offset& operator=(time_offset&&)      = default;
+};
+
+bool operator==(const time_offset& lhs, const time_offset& rhs);
+bool operator!=(const time_offset& lhs, const time_offset& rhs);
+bool operator< (const time_offset& lhs, const time_offset& rhs);
+bool operator<=(const time_offset& lhs, const time_offset& rhs);
+bool operator> (const time_offset& lhs, const time_offset& rhs);
+bool operator>=(const time_offset& lhs, const time_offset& rhs);
+
+std::ostream& operator<<(std::ostream& os, const time_offset& offset);
+
+std::string to_string(const time_offset& offset);
+
+// -----------------------------------------------------------------------------
+
+struct local_datetime
+{
+    local_date date{};
+    local_time time{};
+
+    local_datetime(local_date d, local_time t): date{d}, time{t} {}
+
+    explicit local_datetime(const std::tm& t): date{t}, time{t}{}
+
+    explicit local_datetime(const std::chrono::system_clock::time_point& tp);
+    explicit local_datetime(const std::time_t t);
+
+    operator std::chrono::system_clock::time_point() const;
+    operator std::time_t() const;
+
+    local_datetime() = default;
+    ~local_datetime() = default;
+    local_datetime(local_datetime const&) = default;
+    local_datetime(local_datetime&&)      = default;
+    local_datetime& operator=(local_datetime const&) = default;
+    local_datetime& operator=(local_datetime&&)      = default;
+};
+
+bool operator==(const local_datetime& lhs, const local_datetime& rhs);
+bool operator!=(const local_datetime& lhs, const local_datetime& rhs);
+bool operator< (const local_datetime& lhs, const local_datetime& rhs);
+bool operator<=(const local_datetime& lhs, const local_datetime& rhs);
+bool operator> (const local_datetime& lhs, const local_datetime& rhs);
+bool operator>=(const local_datetime& lhs, const local_datetime& rhs);
+
+std::ostream& operator<<(std::ostream& os, const local_datetime& dt);
+
+std::string to_string(const local_datetime& dt);
+
+// -----------------------------------------------------------------------------
+
+struct offset_datetime
+{
+    local_date  date{};
+    local_time  time{};
+    time_offset offset{};
+
+    offset_datetime(local_date d, local_time t, time_offset o)
+        : date{d}, time{t}, offset{o}
+    {}
+    offset_datetime(const local_datetime& dt, time_offset o)
+        : date{dt.date}, time{dt.time}, offset{o}
+    {}
+    // use the current local timezone offset
+    explicit offset_datetime(const local_datetime& ld);
+    explicit offset_datetime(const std::chrono::system_clock::time_point& tp);
+    explicit offset_datetime(const std::time_t& t);
+    explicit offset_datetime(const std::tm& t);
+
+    operator std::chrono::system_clock::time_point() const;
+
+    operator std::time_t() const;
+
+    offset_datetime() = default;
+    ~offset_datetime() = default;
+    offset_datetime(offset_datetime const&) = default;
+    offset_datetime(offset_datetime&&)      = default;
+    offset_datetime& operator=(offset_datetime const&) = default;
+    offset_datetime& operator=(offset_datetime&&)      = default;
+
+  private:
+
+    static time_offset get_local_offset(const std::time_t* tp);
+};
+
+bool operator==(const offset_datetime& lhs, const offset_datetime& rhs);
+bool operator!=(const offset_datetime& lhs, const offset_datetime& rhs);
+bool operator< (const offset_datetime& lhs, const offset_datetime& rhs);
+bool operator<=(const offset_datetime& lhs, const offset_datetime& rhs);
+bool operator> (const offset_datetime& lhs, const offset_datetime& rhs);
+bool operator>=(const offset_datetime& lhs, const offset_datetime& rhs);
+
+std::ostream& operator<<(std::ostream& os, const offset_datetime& dt);
+
+std::string to_string(const offset_datetime& dt);
+
+}//toml
+#endif // TOML11_DATETIME_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/error_info_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/error_info_fwd.hpp
new file mode 100644
index 00000000..5b8600cb
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/error_info_fwd.hpp
@@ -0,0 +1,97 @@
+#ifndef TOML11_ERROR_INFO_FWD_HPP
+#define TOML11_ERROR_INFO_FWD_HPP
+
+#include "../source_location.hpp"
+#include "../utility.hpp"
+
+namespace toml
+{
+
+// error info returned from parser.
+struct error_info
+{
+    error_info(std::string t, source_location l, std::string m, std::string s = "")
+        : title_(std::move(t)), locations_{std::make_pair(std::move(l), std::move(m))},
+          suffix_(std::move(s))
+    {}
+
+    error_info(std::string t, std::vector<std::pair<source_location, std::string>> l,
+               std::string s = "")
+        : title_(std::move(t)), locations_(std::move(l)), suffix_(std::move(s))
+    {}
+
+    std::string const& title() const noexcept {return title_;}
+    std::string &      title()       noexcept {return title_;}
+
+    std::vector<std::pair<source_location, std::string>> const&
+    locations() const noexcept {return locations_;}
+
+    void add_locations(source_location loc, std::string msg) noexcept
+    {
+        locations_.emplace_back(std::move(loc), std::move(msg));
+    }
+
+    std::string const& suffix() const noexcept {return suffix_;}
+    std::string &      suffix()       noexcept {return suffix_;}
+
+  private:
+
+    std::string title_;
+    std::vector<std::pair<source_location, std::string>> locations_;
+    std::string suffix_; // hint or something like that
+};
+
+// forward decl
+template<typename TypeConfig>
+class basic_value;
+
+namespace detail
+{
+inline error_info make_error_info_rec(error_info e)
+{
+    return e;
+}
+inline error_info make_error_info_rec(error_info e, std::string s)
+{
+    e.suffix() = s;
+    return e;
+}
+
+template<typename TC, typename ... Ts>
+error_info make_error_info_rec(error_info e,
+        const basic_value<TC>& v, std::string msg, Ts&& ... tail);
+
+template<typename ... Ts>
+error_info make_error_info_rec(error_info e,
+        source_location loc, std::string msg, Ts&& ... tail)
+{
+    e.add_locations(std::move(loc), std::move(msg));
+    return make_error_info_rec(std::move(e), std::forward<Ts>(tail)...);
+}
+
+} // detail
+
+template<typename ... Ts>
+error_info make_error_info(
+    std::string title, source_location loc, std::string msg, Ts&& ... tail)
+{
+    error_info ei(std::move(title), std::move(loc), std::move(msg));
+    return detail::make_error_info_rec(ei, std::forward<Ts>(tail) ... );
+}
+
+std::string format_error(const std::string& errkind, const error_info& err);
+std::string format_error(const error_info& err);
+
+// for custom error message
+template<typename ... Ts>
+std::string format_error(std::string title,
+        source_location loc, std::string msg, Ts&& ... tail)
+{
+    return format_error("", make_error_info(std::move(title),
+                std::move(loc), std::move(msg), std::forward<Ts>(tail)...));
+}
+
+std::ostream& operator<<(std::ostream& os, const error_info& e);
+
+} // toml
+#endif // TOML11_ERROR_INFO_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/format_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/format_fwd.hpp
new file mode 100644
index 00000000..d478d96f
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/format_fwd.hpp
@@ -0,0 +1,250 @@
+#ifndef TOML11_FORMAT_FWD_HPP
+#define TOML11_FORMAT_FWD_HPP
+
+#include <iosfwd>
+#include <string>
+#include <utility>
+
+#include <cstddef>
+#include <cstdint>
+
+namespace toml
+{
+
+// toml types with serialization info
+
+enum class indent_char : std::uint8_t
+{
+    space, // use space
+    tab,   // use tab
+    none   // no indent
+};
+
+std::ostream& operator<<(std::ostream& os, const indent_char& c);
+std::string to_string(const indent_char c);
+
+// ----------------------------------------------------------------------------
+// boolean
+
+struct boolean_format_info
+{
+    // nothing, for now
+};
+
+inline bool operator==(const boolean_format_info&, const boolean_format_info&) noexcept
+{
+    return true;
+}
+inline bool operator!=(const boolean_format_info&, const boolean_format_info&) noexcept
+{
+    return false;
+}
+
+// ----------------------------------------------------------------------------
+// integer
+
+enum class integer_format : std::uint8_t
+{
+    dec = 0,
+    bin = 1,
+    oct = 2,
+    hex = 3,
+};
+
+std::ostream& operator<<(std::ostream& os, const integer_format f);
+std::string to_string(const integer_format);
+
+struct integer_format_info
+{
+    integer_format fmt = integer_format::dec;
+    bool        uppercase = true; // hex with uppercase
+    std::size_t width     = 0;    // minimal width (may exceed)
+    std::size_t spacer    = 0;    // position of `_` (if 0, no spacer)
+    std::string suffix    = "";   // _suffix (library extension)
+};
+
+bool operator==(const integer_format_info&, const integer_format_info&) noexcept;
+bool operator!=(const integer_format_info&, const integer_format_info&) noexcept;
+
+// ----------------------------------------------------------------------------
+// floating
+
+enum class floating_format : std::uint8_t
+{
+    defaultfloat = 0,
+    fixed        = 1, // does not include exponential part
+    scientific   = 2, // always include exponential part
+    hex          = 3  // hexfloat extension
+};
+
+std::ostream& operator<<(std::ostream& os, const floating_format f);
+std::string to_string(const floating_format);
+
+struct floating_format_info
+{
+    floating_format fmt = floating_format::defaultfloat;
+    std::size_t prec  = 0;        // precision (if 0, use the default)
+    std::string suffix = "";      // 1.0e+2_suffix (library extension)
+};
+
+bool operator==(const floating_format_info&, const floating_format_info&) noexcept;
+bool operator!=(const floating_format_info&, const floating_format_info&) noexcept;
+
+// ----------------------------------------------------------------------------
+// string
+
+enum class string_format : std::uint8_t
+{
+    basic             = 0,
+    literal           = 1,
+    multiline_basic   = 2,
+    multiline_literal = 3
+};
+
+std::ostream& operator<<(std::ostream& os, const string_format f);
+std::string to_string(const string_format);
+
+struct string_format_info
+{
+    string_format fmt = string_format::basic;
+    bool start_with_newline    = false;
+};
+
+bool operator==(const string_format_info&, const string_format_info&) noexcept;
+bool operator!=(const string_format_info&, const string_format_info&) noexcept;
+
+// ----------------------------------------------------------------------------
+// datetime
+
+enum class datetime_delimiter_kind : std::uint8_t
+{
+    upper_T = 0,
+    lower_t = 1,
+    space   = 2,
+};
+std::ostream& operator<<(std::ostream& os, const datetime_delimiter_kind d);
+std::string to_string(const datetime_delimiter_kind);
+
+struct offset_datetime_format_info
+{
+    datetime_delimiter_kind delimiter = datetime_delimiter_kind::upper_T;
+    bool has_seconds = true;
+    std::size_t subsecond_precision = 6; // [us]
+};
+
+bool operator==(const offset_datetime_format_info&, const offset_datetime_format_info&) noexcept;
+bool operator!=(const offset_datetime_format_info&, const offset_datetime_format_info&) noexcept;
+
+struct local_datetime_format_info
+{
+    datetime_delimiter_kind delimiter = datetime_delimiter_kind::upper_T;
+    bool has_seconds = true;
+    std::size_t subsecond_precision = 6; // [us]
+};
+
+bool operator==(const local_datetime_format_info&, const local_datetime_format_info&) noexcept;
+bool operator!=(const local_datetime_format_info&, const local_datetime_format_info&) noexcept;
+
+struct local_date_format_info
+{
+    // nothing, for now
+};
+
+bool operator==(const local_date_format_info&, const local_date_format_info&) noexcept;
+bool operator!=(const local_date_format_info&, const local_date_format_info&) noexcept;
+
+struct local_time_format_info
+{
+    bool has_seconds = true;
+    std::size_t subsecond_precision = 6; // [us]
+};
+
+bool operator==(const local_time_format_info&, const local_time_format_info&) noexcept;
+bool operator!=(const local_time_format_info&, const local_time_format_info&) noexcept;
+
+// ----------------------------------------------------------------------------
+// array
+
+enum class array_format : std::uint8_t
+{
+    default_format  = 0,
+    oneline         = 1,
+    multiline       = 2,
+    array_of_tables = 3 // [[format.in.this.way]]
+};
+
+std::ostream& operator<<(std::ostream& os, const array_format f);
+std::string to_string(const array_format);
+
+struct array_format_info
+{
+    array_format fmt            = array_format::default_format;
+    indent_char  indent_type    = indent_char::space;
+    std::int32_t body_indent    = 4; // indent in case of multiline
+    std::int32_t closing_indent = 0; // indent of `]`
+};
+
+bool operator==(const array_format_info&, const array_format_info&) noexcept;
+bool operator!=(const array_format_info&, const array_format_info&) noexcept;
+
+// ----------------------------------------------------------------------------
+// table
+
+enum class table_format : std::uint8_t
+{
+    multiline         = 0, // [foo] \n bar = "baz"
+    oneline           = 1, // foo = {bar = "baz"}
+    dotted            = 2, // foo.bar = "baz"
+    multiline_oneline = 3, // foo = { \n bar = "baz" \n }
+    implicit          = 4  // [x] defined by [x.y.z]. skip in serializer.
+};
+
+std::ostream& operator<<(std::ostream& os, const table_format f);
+std::string to_string(const table_format);
+
+struct table_format_info
+{
+    table_format fmt = table_format::multiline;
+    indent_char  indent_type    = indent_char::space;
+    std::int32_t body_indent    = 0; // indent of values
+    std::int32_t name_indent    = 0; // indent of [table]
+    std::int32_t closing_indent = 0; // in case of {inline-table}
+};
+
+bool operator==(const table_format_info&, const table_format_info&) noexcept;
+bool operator!=(const table_format_info&, const table_format_info&) noexcept;
+
+// ----------------------------------------------------------------------------
+// wrapper
+
+namespace detail
+{
+template<typename T, typename F>
+struct value_with_format
+{
+    using value_type  = T;
+    using format_type = F;
+
+    value_with_format()  = default;
+    ~value_with_format() = default;
+    value_with_format(const value_with_format&) = default;
+    value_with_format(value_with_format&&)      = default;
+    value_with_format& operator=(const value_with_format&) = default;
+    value_with_format& operator=(value_with_format&&)      = default;
+
+    value_with_format(value_type v, format_type f)
+        : value{std::move(v)}, format{std::move(f)}
+    {}
+
+    template<typename U>
+    value_with_format(value_with_format<U, format_type> other)
+        : value{std::move(other.value)}, format{std::move(other.format)}
+    {}
+
+    value_type  value;
+    format_type format;
+};
+} // detail
+
+} // namespace toml
+#endif // TOML11_FORMAT_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/literal_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/literal_fwd.hpp
new file mode 100644
index 00000000..e46612ce
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/literal_fwd.hpp
@@ -0,0 +1,33 @@
+#ifndef TOML11_LITERAL_FWD_HPP
+#define TOML11_LITERAL_FWD_HPP
+
+#include "../location.hpp"
+#include "../types.hpp"
+#include "../version.hpp" // IWYU pragma: keep for TOML11_HAS_CHAR8_T
+
+namespace toml
+{
+
+namespace detail
+{
+// implementation
+::toml::value literal_internal_impl(location loc);
+} // detail
+
+inline namespace literals
+{
+inline namespace toml_literals
+{
+
+::toml::value operator"" _toml(const char* str, std::size_t len);
+
+#if defined(TOML11_HAS_CHAR8_T)
+// value of u8"" literal has been changed from char to char8_t and char8_t is
+// NOT compatible to char
+::toml::value operator"" _toml(const char8_t* str, std::size_t len);
+#endif
+
+} // toml_literals
+} // literals
+} // toml
+#endif // TOML11_LITERAL_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/location_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/location_fwd.hpp
new file mode 100644
index 00000000..f6dbfdbc
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/location_fwd.hpp
@@ -0,0 +1,145 @@
+#ifndef TOML11_LOCATION_FWD_HPP
+#define TOML11_LOCATION_FWD_HPP
+
+#include "../result.hpp"
+
+#include <memory>
+#include <string>
+#include <vector>
+
+namespace toml
+{
+namespace detail
+{
+
+class region; // fwd decl
+
+//
+// To represent where we are reading in the parse functions.
+// Since it "points" somewhere in the input stream, the length is always 1.
+//
+class location
+{
+  public:
+
+    using char_type       = unsigned char; // must be unsigned
+    using container_type  = std::vector<char_type>;
+    using difference_type = typename container_type::difference_type; // to suppress sign-conversion warning
+    using source_ptr      = std::shared_ptr<const container_type>;
+
+  public:
+
+    location(source_ptr src, std::string src_name)
+        : source_(std::move(src)), source_name_(std::move(src_name)),
+          location_(0), line_number_(1)
+    {}
+
+    location(const location&) = default;
+    location(location&&)      = default;
+    location& operator=(const location&) = default;
+    location& operator=(location&&)      = default;
+    ~location() = default;
+
+    void advance(std::size_t n = 1) noexcept;
+    void retrace(std::size_t n = 1) noexcept;
+
+    bool is_ok() const noexcept { return static_cast<bool>(this->source_); }
+
+    bool eof() const noexcept;
+    char_type current() const;
+
+    char_type peek();
+
+    std::size_t get_location() const noexcept
+    {
+        return this->location_;
+    }
+    void set_location(const std::size_t loc) noexcept;
+
+    std::size_t line_number() const noexcept
+    {
+        return this->line_number_;
+    }
+    std::string get_line() const;
+    std::size_t column_number() const noexcept;
+
+    source_ptr const&  source()      const noexcept {return this->source_;}
+    std::string const& source_name() const noexcept {return this->source_name_;}
+
+  private:
+
+    void advance_line_number(const std::size_t n);
+    void retrace_line_number(const std::size_t n);
+
+  private:
+
+    friend region;
+
+  private:
+
+    source_ptr  source_;
+    std::string source_name_;
+    std::size_t location_; // std::vector<>::difference_type is signed
+    std::size_t line_number_;
+};
+
+bool operator==(const location& lhs, const location& rhs) noexcept;
+bool operator!=(const location& lhs, const location& rhs);
+
+location prev(const location& loc);
+location next(const location& loc);
+location make_temporary_location(const std::string& str) noexcept;
+
+template<typename F>
+result<location, none_t>
+find_if(const location& first, const location& last, const F& func) noexcept
+{
+    if(first.source() != last.source())             { return err(); }
+    if(first.get_location() >= last.get_location()) { return err(); }
+
+    auto loc = first;
+    while(loc.get_location() != last.get_location())
+    {
+        if(func(loc.current()))
+        {
+            return ok(loc);
+        }
+        loc.advance();
+    }
+    return err();
+}
+
+template<typename F>
+result<location, none_t>
+rfind_if(location first, const location& last, const F& func)
+{
+    if(first.source() != last.source())             { return err(); }
+    if(first.get_location() >= last.get_location()) { return err(); }
+
+    auto loc = last;
+    while(loc.get_location() != first.get_location())
+    {
+        if(func(loc.current()))
+        {
+            return ok(loc);
+        }
+        loc.retrace();
+    }
+    if(func(first.current()))
+    {
+        return ok(first);
+    }
+    return err();
+}
+
+result<location, none_t> find(const location& first, const location& last,
+                              const location::char_type val);
+result<location, none_t> rfind(const location& first, const location& last,
+                               const location::char_type val);
+
+std::size_t count(const location& first, const location& last,
+                  const location::char_type& c);
+
+} // detail
+} // toml
+#endif // TOML11_LOCATION_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/region_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/region_fwd.hpp
new file mode 100644
index 00000000..b40317d2
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/region_fwd.hpp
@@ -0,0 +1,104 @@
+#ifndef TOML11_REGION_FWD_HPP
+#define TOML11_REGION_FWD_HPP
+
+#include "../location.hpp"
+
+#include <string>
+#include <vector>
+
+#include <cassert>
+
+namespace toml
+{
+namespace detail
+{
+
+//
+// To represent where is a toml::value defined, or where does an error occur.
+// Stored in toml::value. source_location will be constructed based on this.
+//
+class region
+{
+  public:
+
+    using char_type       = location::char_type;
+    using container_type  = location::container_type;
+    using difference_type = location::difference_type;
+    using source_ptr      = location::source_ptr;
+
+    using iterator       = typename container_type::iterator;
+    using const_iterator = typename container_type::const_iterator;
+
+  public:
+
+    // a value that is constructed manually does not have input stream info
+    region()
+        : source_(nullptr), source_name_(""), length_(0),
+          first_line_(0), first_column_(0), last_line_(0), last_column_(0)
+    {}
+
+    // a value defined in [first, last).
+    // Those source must be the same. Instread, `region` does not make sense.
+    region(const location& first, const location& last);
+
+    // shorthand of [loc, loc+1)
+    explicit region(const location& loc);
+
+    ~region() = default;
+    region(const region&) = default;
+    region(region&&)      = default;
+    region& operator=(const region&) = default;
+    region& operator=(region&&)      = default;
+
+    bool is_ok() const noexcept { return static_cast<bool>(this->source_); }
+
+    operator bool() const noexcept { return this->is_ok(); }
+
+    std::size_t length() const noexcept {return this->length_;}
+
+    std::size_t first_line_number() const noexcept
+    {
+        return this->first_line_;
+    }
+    std::size_t first_column_number() const noexcept
+    {
+        return this->first_column_;
+    }
+    std::size_t last_line_number() const noexcept
+    {
+        return this->last_line_;
+    }
+    std::size_t last_column_number() const noexcept
+    {
+        return this->last_column_;
+    }
+
+    char_type at(std::size_t i) const;
+
+    const_iterator begin() const noexcept;
+    const_iterator end() const noexcept;
+    const_iterator cbegin() const noexcept;
+    const_iterator cend() const noexcept;
+
+    std::string as_string() const;
+    std::vector<std::string> as_lines() const;
+
+    source_ptr const&  source()      const noexcept {return this->source_;}
+    std::string const& source_name() const noexcept {return this->source_name_;}
+
+  private:
+
+    source_ptr  source_;
+    std::string source_name_;
+    std::size_t length_;
+    std::size_t first_;
+    std::size_t first_line_;
+    std::size_t first_column_;
+    std::size_t last_;
+    std::size_t last_line_;
+    std::size_t last_column_;
+};
+
+} // namespace detail
+} // namespace toml
+#endif // TOML11_REGION_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/scanner_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/scanner_fwd.hpp
new file mode 100644
index 00000000..a8867277
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/scanner_fwd.hpp
@@ -0,0 +1,391 @@
+#ifndef TOML11_SCANNER_FWD_HPP
+#define TOML11_SCANNER_FWD_HPP
+
+#include "../region.hpp"
+
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include <cassert>
+#include <cstdio>
+#include <cctype>
+
+namespace toml
+{
+namespace detail
+{
+
+class scanner_base
+{
+  public:
+    virtual ~scanner_base() = default;
+    virtual region scan(location& loc) const = 0;
+    virtual scanner_base* clone() const = 0;
+
+    // returns expected character or set of characters or literal.
+    // to show the error location, it changes loc (in `sequence`, especially).
+    virtual std::string expected_chars(location& loc) const = 0;
+    virtual std::string name() const = 0;
+};
+
+// make `scanner*` copyable
+struct scanner_storage
+{
+    template<typename Scanner, cxx::enable_if_t<
+        std::is_base_of<scanner_base, cxx::remove_cvref_t<Scanner>>::value,
+        std::nullptr_t> = nullptr>
+    explicit scanner_storage(Scanner&& s)
+        : scanner_(cxx::make_unique<cxx::remove_cvref_t<Scanner>>(std::forward<Scanner>(s)))
+    {}
+    ~scanner_storage() = default;
+
+    scanner_storage(const scanner_storage& other);
+    scanner_storage& operator=(const scanner_storage& other);
+    scanner_storage(scanner_storage&&) = default;
+    scanner_storage& operator=(scanner_storage&&) = default;
+
+    bool is_ok() const noexcept {return static_cast<bool>(scanner_);}
+
+    region scan(location& loc) const;
+
+    std::string expected_chars(location& loc) const;
+
+    scanner_base& get() const noexcept;
+
+    std::string name() const;
+
+  private:
+
+    std::unique_ptr<scanner_base> scanner_;
+};
+
+// ----------------------------------------------------------------------------
+
+class character final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  public:
+
+    explicit character(const char_type c) noexcept
+        : value_(c)
+    {}
+    ~character() override = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location&) const override;
+
+    scanner_base* clone() const override;
+
+    std::string name() const override;
+
+  private:
+    char_type value_;
+};
+
+// ----------------------------------------------------------------------------
+
+class character_either final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  public:
+
+    explicit character_either(std::initializer_list<char_type> cs) noexcept
+        : chars_(std::move(cs))
+    {
+        assert(! this->chars_.empty());
+    }
+
+    template<std::size_t N>
+    explicit character_either(const char (&cs)[N]) noexcept
+        : chars_(N-1, '\0')
+    {
+        static_assert(N >= 1, "");
+        for(std::size_t i=0; i+1<N; ++i)
+        {
+            chars_.at(i) = char_type(cs[i]);
+        }
+    }
+    ~character_either() override = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location&) const override;
+
+    scanner_base* clone() const override;
+
+    void push_back(const char_type c);
+
+    std::string name() const override;
+
+  private:
+    std::vector<char_type> chars_;
+};
+
+// ----------------------------------------------------------------------------
+
+class character_in_range final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  public:
+
+    explicit character_in_range(const char_type from, const char_type to) noexcept
+        : from_(from), to_(to)
+    {}
+    ~character_in_range() override = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location&) const override;
+
+    scanner_base* clone() const override;
+
+    std::string name() const override;
+
+  private:
+    char_type from_;
+    char_type to_;
+};
+
+// ----------------------------------------------------------------------------
+
+class literal final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  public:
+
+    template<std::size_t N>
+    explicit literal(const char (&cs)[N]) noexcept
+        : value_(cs), size_(N-1) // remove null character at the end
+    {}
+    ~literal() override = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location&) const override;
+
+    scanner_base* clone() const override;
+
+    std::string name() const override;
+
+  private:
+    const char* value_;
+    std::size_t size_;
+};
+
+// ----------------------------------------------------------------------------
+
+class sequence final: public scanner_base
+{
+  public:
+    using char_type = location::char_type;
+
+  public:
+
+    template<typename ... Ts>
+    explicit sequence(Ts&& ... args)
+    {
+        push_back_all(std::forward<Ts>(args)...);
+    }
+    sequence(const sequence&)            = default;
+    sequence(sequence&&)                 = default;
+    sequence& operator=(const sequence&) = default;
+    sequence& operator=(sequence&&)      = default;
+    ~sequence() override                 = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location& loc) const override;
+
+    scanner_base* clone() const override;
+
+    template<typename Scanner>
+    void push_back(Scanner&& other_scanner)
+    {
+        this->others_.emplace_back(std::forward<Scanner>(other_scanner));
+    }
+
+    std::string name() const override;
+
+  private:
+
+    void push_back_all()
+    {
+        return;
+    }
+    template<typename T, typename ... Ts>
+    void push_back_all(T&& head, Ts&& ... args)
+    {
+        others_.emplace_back(std::forward<T>(head));
+        push_back_all(std::forward<Ts>(args)...);
+        return;
+    }
+
+  private:
+    std::vector<scanner_storage> others_;
+};
+
+// ----------------------------------------------------------------------------
+
+class either final: public scanner_base
+{
+  public:
+    using char_type = location::char_type;
+
+  public:
+
+    template<typename ... Ts>
+    explicit either(Ts&& ... args)
+    {
+        push_back_all(std::forward<Ts>(args)...);
+    }
+    either(const either&)            = default;
+    either(either&&)                 = default;
+    either& operator=(const either&) = default;
+    either& operator=(either&&)      = default;
+    ~either() override               = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location& loc) const override;
+
+    scanner_base* clone() const override;
+
+    template<typename Scanner>
+    void push_back(Scanner&& other_scanner)
+    {
+        this->others_.emplace_back(std::forward<Scanner>(other_scanner));
+    }
+
+    std::string name() const override;
+
+  private:
+
+    void push_back_all()
+    {
+        return;
+    }
+    template<typename T, typename ... Ts>
+    void push_back_all(T&& head, Ts&& ... args)
+    {
+        others_.emplace_back(std::forward<T>(head));
+        push_back_all(std::forward<Ts>(args)...);
+        return;
+    }
+
+  private:
+    std::vector<scanner_storage> others_;
+};
+
+// ----------------------------------------------------------------------------
+
+class repeat_exact final: public scanner_base
+{
+  public:
+    using char_type = location::char_type;
+
+  public:
+
+    template<typename Scanner>
+    repeat_exact(const std::size_t length, Scanner&& other)
+        : length_(length), other_(std::forward<Scanner>(other))
+    {}
+    repeat_exact(const repeat_exact&)            = default;
+    repeat_exact(repeat_exact&&)                 = default;
+    repeat_exact& operator=(const repeat_exact&) = default;
+    repeat_exact& operator=(repeat_exact&&)      = default;
+    ~repeat_exact() override                     = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location& loc) const override;
+
+    scanner_base* clone() const override;
+
+    std::string name() const override;
+
+  private:
+    std::size_t length_;
+    scanner_storage other_;
+};
+
+// ----------------------------------------------------------------------------
+
+class repeat_at_least final: public scanner_base
+{
+  public:
+    using char_type = location::char_type;
+
+  public:
+
+    template<typename Scanner>
+    repeat_at_least(const std::size_t length, Scanner&& s)
+        : length_(length), other_(std::forward<Scanner>(s))
+    {}
+    repeat_at_least(const repeat_at_least&)            = default;
+    repeat_at_least(repeat_at_least&&)                 = default;
+    repeat_at_least& operator=(const repeat_at_least&) = default;
+    repeat_at_least& operator=(repeat_at_least&&)      = default;
+    ~repeat_at_least() override                        = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location& loc) const override;
+
+    scanner_base* clone() const override;
+
+    std::string name() const override;
+
+  private:
+    std::size_t length_;
+    scanner_storage other_;
+};
+
+// ----------------------------------------------------------------------------
+
+class maybe final: public scanner_base
+{
+  public:
+    using char_type = location::char_type;
+
+  public:
+
+    template<typename Scanner>
+    explicit maybe(Scanner&& s)
+        : other_(std::forward<Scanner>(s))
+    {}
+    maybe(const maybe&)            = default;
+    maybe(maybe&&)                 = default;
+    maybe& operator=(const maybe&) = default;
+    maybe& operator=(maybe&&)      = default;
+    ~maybe() override              = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location&) const override;
+
+    scanner_base* clone() const override;
+
+    std::string name() const override;
+
+  private:
+    scanner_storage other_;
+};
+
+} // detail
+} // toml
+#endif // TOML11_SCANNER_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/source_location_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/source_location_fwd.hpp
new file mode 100644
index 00000000..77c7579d
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/source_location_fwd.hpp
@@ -0,0 +1,115 @@
+#ifndef TOML11_SOURCE_LOCATION_FWD_HPP
+#define TOML11_SOURCE_LOCATION_FWD_HPP
+
+#include "../region.hpp"
+
+#include <sstream>
+#include <string>
+#include <vector>
+
+namespace toml
+{
+
+// A struct to contain location in a toml file.
+struct source_location
+{
+  public:
+
+    explicit source_location(const detail::region& r);
+    ~source_location() = default;
+    source_location(source_location const&) = default;
+    source_location(source_location &&)     = default;
+    source_location& operator=(source_location const&) = default;
+    source_location& operator=(source_location &&)     = default;
+
+    bool is_ok() const noexcept {return this->is_ok_;}
+    std::size_t length() const noexcept {return this->length_;}
+
+    std::size_t first_line_number()   const noexcept {return this->first_line_;}
+    std::size_t first_column_number() const noexcept {return this->first_column_;}
+    std::size_t last_line_number()    const noexcept {return this->last_line_;}
+    std::size_t last_column_number()  const noexcept {return this->last_column_;}
+
+    std::string const& file_name()  const noexcept {return this->file_name_;}
+
+    std::size_t num_lines() const noexcept {return this->line_str_.size();}
+
+    std::string const& first_line() const;
+    std::string const& last_line() const;
+
+    std::vector<std::string> const& lines() const noexcept {return line_str_;}
+
+  private:
+
+    bool        is_ok_;
+    std::size_t first_line_;
+    std::size_t first_column_;
+    std::size_t last_line_;
+    std::size_t last_column_;
+    std::size_t length_;
+    std::string file_name_;
+    std::vector<std::string> line_str_;
+};
+
+namespace detail
+{
+
+std::size_t integer_width_base10(std::size_t i) noexcept;
+
+inline std::size_t line_width() noexcept {return 0;}
+
+template<typename ... Ts>
+std::size_t line_width(const source_location& loc, const std::string& /*msg*/,
+        const Ts& ... tail) noexcept
+{
+    return (std::max)(
+            integer_width_base10(loc.last_line_number()), line_width(tail...));
+}
+
+std::ostringstream&
+format_filename(std::ostringstream& oss, const source_location& loc);
+
+std::ostringstream&
+format_empty_line(std::ostringstream& oss, const std::size_t lnw);
+
+std::ostringstream& format_line(std::ostringstream& oss,
+    const std::size_t lnw, const std::size_t linenum, const std::string& line);
+
+std::ostringstream& format_underline(std::ostringstream& oss,
+        const std::size_t lnw, const std::size_t col, const std::size_t len,
+        const std::string& msg);
+
+std::string format_location_impl(const std::size_t lnw,
+    const std::string& prev_fname,
+    const source_location& loc, const std::string& msg);
+
+inline std::string format_location_rec(const std::size_t, const std::string&)
+{
+    return "";
+}
+
+template<typename ... Ts>
+std::string format_location_rec(const std::size_t lnw,
+        const std::string& prev_fname,
+        const source_location& loc, const std::string& msg,
+        const Ts& ... tail)
+{
+    return format_location_impl(lnw, prev_fname, loc, msg) +
+           format_location_rec(lnw, loc.file_name(), tail...);
+}
+
+} // namespace detail
+
+// format a location info without title
+template<typename ... Ts>
+std::string format_location(
+        const source_location& loc, const std::string& msg, const Ts& ... tail)
+{
+    const auto lnw = detail::line_width(loc, msg, tail...);
+
+    const std::string f(""); // at the 1st iteration, no prev_filename is given
+    return detail::format_location_rec(lnw, f, loc, msg, tail...);
+}
+
+} // toml
+#endif // TOML11_SOURCE_LOCATION_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/syntax_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/syntax_fwd.hpp
new file mode 100644
index 00000000..35608215
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/syntax_fwd.hpp
@@ -0,0 +1,357 @@
+#ifndef TOML11_SYNTAX_FWD_HPP
+#define TOML11_SYNTAX_FWD_HPP
+
+#include "../scanner.hpp"
+#include "../spec.hpp"
+
+namespace toml
+{
+namespace detail
+{
+namespace syntax
+{
+
+using char_type = location::char_type;
+
+// ===========================================================================
+// UTF-8
+
+// avoid redundant representation and out-of-unicode sequence
+
+character_in_range utf8_1byte (const spec&);
+sequence           utf8_2bytes(const spec&);
+sequence           utf8_3bytes(const spec&);
+sequence           utf8_4bytes(const spec&);
+
+class non_ascii final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  public:
+
+    explicit non_ascii(const spec& s) noexcept;
+    ~non_ascii() override = default;
+
+    region scan(location& loc) const override
+    {
+        return scanner_.scan(loc);
+    }
+
+    std::string expected_chars(location&) const override
+    {
+        return "non-ascii utf-8 bytes";
+    }
+
+    scanner_base* clone() const override
+    {
+        return new non_ascii(*this);
+    }
+
+    std::string name() const override
+    {
+        return "non_ascii";
+    }
+
+  private:
+
+    either scanner_;
+};
+
+// ===========================================================================
+// Whitespace
+
+character_either wschar(const spec&);
+
+repeat_at_least ws(const spec& s);
+
+// ===========================================================================
+// Newline
+
+either newline(const spec&);
+
+// ===========================================================================
+// Comments
+
+either allowed_comment_char(const spec& s);
+
+// XXX Note that it does not take newline
+sequence comment(const spec& s);
+
+// ===========================================================================
+// Boolean
+
+either boolean(const spec&);
+
+// ===========================================================================
+// Integer
+
+class digit final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  public:
+
+    explicit digit(const spec&) noexcept;
+    ~digit() override = default;
+
+    region scan(location& loc) const override
+    {
+        return scanner_.scan(loc);
+    }
+
+    std::string expected_chars(location&) const override
+    {
+        return "digit [0-9]";
+    }
+
+    scanner_base* clone() const override
+    {
+        return new digit(*this);
+    }
+
+    std::string name() const override
+    {
+        return "digit";
+    }
+
+  private:
+
+    character_in_range scanner_;
+};
+
+class alpha final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  public:
+
+    explicit alpha(const spec&) noexcept;
+    ~alpha() override = default;
+
+    region scan(location& loc) const override
+    {
+        return scanner_.scan(loc);
+    }
+
+    std::string expected_chars(location&) const override
+    {
+        return "alpha [a-zA-Z]";
+    }
+
+    scanner_base* clone() const override
+    {
+        return new alpha(*this);
+    }
+
+    std::string name() const override
+    {
+        return "alpha";
+    }
+
+  private:
+
+    either scanner_;
+};
+
+class hexdig final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  public:
+
+    explicit hexdig(const spec& s) noexcept;
+    ~hexdig() override = default;
+
+    region scan(location& loc) const override
+    {
+        return scanner_.scan(loc);
+    }
+
+    std::string expected_chars(location&) const override
+    {
+        return "hex [0-9a-fA-F]";
+    }
+
+    scanner_base* clone() const override
+    {
+        return new hexdig(*this);
+    }
+
+    std::string name() const override
+    {
+        return "hexdig";
+    }
+
+  private:
+
+    either scanner_;
+};
+
+sequence num_suffix(const spec& s);
+
+sequence dec_int(const spec& s);
+sequence hex_int(const spec& s);
+sequence oct_int(const spec&);
+sequence bin_int(const spec&);
+either   integer(const spec& s);
+
+// ===========================================================================
+// Floating
+
+sequence zero_prefixable_int(const spec& s);
+sequence fractional_part(const spec& s);
+sequence exponent_part(const spec& s);
+sequence hex_floating(const spec& s);
+either   floating(const spec& s);
+
+// ===========================================================================
+// Datetime
+
+sequence local_date(const spec& s);
+sequence local_time(const spec& s);
+either time_offset(const spec& s);
+sequence full_time(const spec& s);
+character_either time_delim(const spec&);
+sequence local_datetime(const spec& s);
+sequence offset_datetime(const spec& s);
+
+// ===========================================================================
+// String
+
+sequence escaped(const spec& s);
+
+either basic_char(const spec& s);
+
+sequence basic_string(const spec& s);
+
+// ---------------------------------------------------------------------------
+// multiline string
+
+sequence escaped_newline(const spec& s);
+sequence ml_basic_string(const spec& s);
+
+// ---------------------------------------------------------------------------
+// literal string
+
+either literal_char(const spec& s);
+sequence literal_string(const spec& s);
+
+sequence ml_literal_string(const spec& s);
+
+either string(const spec& s);
+
+// ===========================================================================
+// Keys
+
+// to keep `expected_chars` simple
+class non_ascii_key_char final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  private:
+
+    using in_range = character_in_range; // make definition short
+
+  public:
+
+    explicit non_ascii_key_char(const spec& s) noexcept;
+    ~non_ascii_key_char() override = default;
+
+    region scan(location& loc) const override;
+
+    std::string expected_chars(location&) const override
+    {
+        return "bare key non-ASCII script";
+    }
+
+    scanner_base* clone() const override
+    {
+        return new non_ascii_key_char(*this);
+    }
+
+    std::string name() const override
+    {
+        return "non-ASCII bare key";
+    }
+
+  private:
+
+    std::uint32_t read_utf8(location& loc) const;
+};
+
+
+repeat_at_least unquoted_key(const spec& s);
+
+either quoted_key(const spec& s);
+
+either simple_key(const spec& s);
+
+sequence dot_sep(const spec& s);
+
+sequence dotted_key(const spec& s);
+
+
+class key final : public scanner_base
+{
+  public:
+
+    using char_type = location::char_type;
+
+  public:
+
+    explicit key(const spec& s) noexcept;
+    ~key() override = default;
+
+    region scan(location& loc) const override
+    {
+        return scanner_.scan(loc);
+    }
+
+    std::string expected_chars(location&) const override
+    {
+        return "basic key([a-zA-Z0-9_-]) or quoted key(\" or ')";
+    }
+
+    scanner_base* clone() const override
+    {
+        return new key(*this);
+    }
+
+    std::string name() const override
+    {
+        return "key";
+    }
+
+  private:
+
+    either scanner_;
+};
+
+sequence keyval_sep(const spec& s);
+
+// ===========================================================================
+// Table key
+
+sequence std_table(const spec& s);
+
+sequence array_table(const spec& s);
+
+// ===========================================================================
+// extension: null
+
+literal null_value(const spec&);
+
+} // namespace syntax
+} // namespace detail
+} // namespace toml
+#endif // TOML11_SYNTAX_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/fwd/value_t_fwd.hpp b/src/frontend/qt_sdl/toml/toml11/fwd/value_t_fwd.hpp
new file mode 100644
index 00000000..fddf8434
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/fwd/value_t_fwd.hpp
@@ -0,0 +1,117 @@
+#ifndef TOML11_VALUE_T_FWD_HPP
+#define TOML11_VALUE_T_FWD_HPP
+
+#include "../compat.hpp"
+#include "../format.hpp"
+
+#include <iosfwd>
+#include <string>
+#include <type_traits>
+
+#include <cstdint>
+
+namespace toml
+{
+
+// forward decl
+template<typename TypeConfig>
+class basic_value;
+
+// ----------------------------------------------------------------------------
+// enum representing toml types
+
+enum class value_t : std::uint8_t
+{
+    empty           =  0,
+    boolean         =  1,
+    integer         =  2,
+    floating        =  3,
+    string          =  4,
+    offset_datetime =  5,
+    local_datetime  =  6,
+    local_date      =  7,
+    local_time      =  8,
+    array           =  9,
+    table           = 10
+};
+
+std::ostream& operator<<(std::ostream& os, value_t t);
+std::string to_string(value_t t);
+
+
+// ----------------------------------------------------------------------------
+// meta functions for internal use
+
+namespace detail
+{
+
+template<value_t V>
+using value_t_constant = std::integral_constant<value_t, V>;
+
+template<typename T, typename Value>
+struct type_to_enum : value_t_constant<value_t::empty> {};
+
+template<typename V> struct type_to_enum<typename V::boolean_type        , V> : value_t_constant<value_t::boolean        > {};
+template<typename V> struct type_to_enum<typename V::integer_type        , V> : value_t_constant<value_t::integer        > {};
+template<typename V> struct type_to_enum<typename V::floating_type       , V> : value_t_constant<value_t::floating       > {};
+template<typename V> struct type_to_enum<typename V::string_type         , V> : value_t_constant<value_t::string         > {};
+template<typename V> struct type_to_enum<typename V::offset_datetime_type, V> : value_t_constant<value_t::offset_datetime> {};
+template<typename V> struct type_to_enum<typename V::local_datetime_type , V> : value_t_constant<value_t::local_datetime > {};
+template<typename V> struct type_to_enum<typename V::local_date_type     , V> : value_t_constant<value_t::local_date     > {};
+template<typename V> struct type_to_enum<typename V::local_time_type     , V> : value_t_constant<value_t::local_time     > {};
+template<typename V> struct type_to_enum<typename V::array_type          , V> : value_t_constant<value_t::array          > {};
+template<typename V> struct type_to_enum<typename V::table_type          , V> : value_t_constant<value_t::table          > {};
+
+template<value_t V, typename Value>
+struct enum_to_type { using type = void; };
+
+template<typename V> struct enum_to_type<value_t::boolean        , V> { using type = typename V::boolean_type        ; };
+template<typename V> struct enum_to_type<value_t::integer        , V> { using type = typename V::integer_type        ; };
+template<typename V> struct enum_to_type<value_t::floating       , V> { using type = typename V::floating_type       ; };
+template<typename V> struct enum_to_type<value_t::string         , V> { using type = typename V::string_type         ; };
+template<typename V> struct enum_to_type<value_t::offset_datetime, V> { using type = typename V::offset_datetime_type; };
+template<typename V> struct enum_to_type<value_t::local_datetime , V> { using type = typename V::local_datetime_type ; };
+template<typename V> struct enum_to_type<value_t::local_date     , V> { using type = typename V::local_date_type     ; };
+template<typename V> struct enum_to_type<value_t::local_time     , V> { using type = typename V::local_time_type     ; };
+template<typename V> struct enum_to_type<value_t::array          , V> { using type = typename V::array_type          ; };
+template<typename V> struct enum_to_type<value_t::table          , V> { using type = typename V::table_type          ; };
+
+template<value_t V, typename Value>
+using enum_to_type_t = typename enum_to_type<V, Value>::type;
+
+template<value_t V>
+struct enum_to_fmt_type { using type = void; };
+
+template<> struct enum_to_fmt_type<value_t::boolean        > { using type = boolean_format_info        ; };
+template<> struct enum_to_fmt_type<value_t::integer        > { using type = integer_format_info        ; };
+template<> struct enum_to_fmt_type<value_t::floating       > { using type = floating_format_info       ; };
+template<> struct enum_to_fmt_type<value_t::string         > { using type = string_format_info         ; };
+template<> struct enum_to_fmt_type<value_t::offset_datetime> { using type = offset_datetime_format_info; };
+template<> struct enum_to_fmt_type<value_t::local_datetime > { using type = local_datetime_format_info ; };
+template<> struct enum_to_fmt_type<value_t::local_date     > { using type = local_date_format_info     ; };
+template<> struct enum_to_fmt_type<value_t::local_time     > { using type = local_time_format_info     ; };
+template<> struct enum_to_fmt_type<value_t::array          > { using type = array_format_info          ; };
+template<> struct enum_to_fmt_type<value_t::table          > { using type = table_format_info          ; };
+
+template<value_t V>
+using enum_to_fmt_type_t = typename enum_to_fmt_type<V>::type;
+
+template<typename T, typename Value>
+struct is_exact_toml_type0 : cxx::disjunction<
+    std::is_same<T, typename Value::boolean_type        >,
+    std::is_same<T, typename Value::integer_type        >,
+    std::is_same<T, typename Value::floating_type       >,
+    std::is_same<T, typename Value::string_type         >,
+    std::is_same<T, typename Value::offset_datetime_type>,
+    std::is_same<T, typename Value::local_datetime_type >,
+    std::is_same<T, typename Value::local_date_type     >,
+    std::is_same<T, typename Value::local_time_type     >,
+    std::is_same<T, typename Value::array_type          >,
+    std::is_same<T, typename Value::table_type          >
+    >{};
+template<typename T, typename V> struct is_exact_toml_type: is_exact_toml_type0<cxx::remove_cvref_t<T>, V> {};
+template<typename T, typename V> struct is_not_toml_type : cxx::negation<is_exact_toml_type<T, V>> {};
+
+} // namespace detail
+} // namespace toml
+#endif // TOML11_VALUE_T_FWD_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/get.hpp b/src/frontend/qt_sdl/toml/toml11/get.hpp
new file mode 100644
index 00000000..3805d09f
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/get.hpp
@@ -0,0 +1,632 @@
+#ifndef TOML11_GET_HPP
+#define TOML11_GET_HPP
+
+#include <algorithm>
+
+#include "from.hpp"
+#include "types.hpp"
+#include "value.hpp"
+
+#if defined(TOML11_HAS_STRING_VIEW)
+#include <string_view>
+#endif // string_view
+
+namespace toml
+{
+
+// ============================================================================
+// T is toml::value; identity transformation.
+
+template<typename T, typename TC>
+cxx::enable_if_t<std::is_same<T, basic_value<TC>>::value, T>&
+get(basic_value<TC>& v)
+{
+    return v;
+}
+
+template<typename T, typename TC>
+cxx::enable_if_t<std::is_same<T, basic_value<TC>>::value, T> const&
+get(const basic_value<TC>& v)
+{
+    return v;
+}
+
+template<typename T, typename TC>
+cxx::enable_if_t<std::is_same<T, basic_value<TC>>::value, T>
+get(basic_value<TC>&& v)
+{
+    return basic_value<TC>(std::move(v));
+}
+
+// ============================================================================
+// exact toml::* type
+
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_exact_toml_type<T, basic_value<TC>>::value, T> &
+get(basic_value<TC>& v)
+{
+    constexpr auto ty = detail::type_to_enum<T, basic_value<TC>>::value;
+    return detail::getter<TC, ty>::get(v);
+}
+
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_exact_toml_type<T, basic_value<TC>>::value, T> const&
+get(const basic_value<TC>& v)
+{
+    constexpr auto ty = detail::type_to_enum<T, basic_value<TC>>::value;
+    return detail::getter<TC, ty>::get(v);
+}
+
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_exact_toml_type<T, basic_value<TC>>::value, T>
+get(basic_value<TC>&& v)
+{
+    constexpr auto ty = detail::type_to_enum<T, basic_value<TC>>::value;
+    return detail::getter<TC, ty>::get(std::move(v));
+}
+
+// ============================================================================
+// T is toml::basic_value<U>
+
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_basic_value<T>,
+    cxx::negation<std::is_same<T, basic_value<TC>>>
+    >::value, T>
+get(basic_value<TC> v)
+{
+    return T(std::move(v));
+}
+
+// ============================================================================
+// integer convertible from toml::value::integer_type
+
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    std::is_integral<T>,
+    cxx::negation<std::is_same<T, bool>>,
+    detail::is_not_toml_type<T, basic_value<TC>>,
+    cxx::negation<detail::has_from_toml_method<T, TC>>,
+    cxx::negation<detail::has_specialized_from<T>>
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    return static_cast<T>(v.as_integer());
+}
+
+// ============================================================================
+// floating point convertible from toml::value::floating_type
+
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    std::is_floating_point<T>,
+    detail::is_not_toml_type<T, basic_value<TC>>,
+    cxx::negation<detail::has_from_toml_method<T, TC>>,
+    cxx::negation<detail::has_specialized_from<T>>
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    return static_cast<T>(v.as_floating());
+}
+
+// ============================================================================
+// std::string with different char/trait/allocator
+
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_not_toml_type<T, basic_value<TC>>,
+    detail::is_1byte_std_basic_string<T>
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    return detail::string_conv<cxx::remove_cvref_t<T>>(v.as_string());
+}
+
+// ============================================================================
+// std::string_view
+
+#if defined(TOML11_HAS_STRING_VIEW)
+
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_string_view_of<T, typename basic_value<TC>::string_type>::value, T>
+get(const basic_value<TC>& v)
+{
+    return T(v.as_string());
+}
+
+#endif // string_view
+
+// ============================================================================
+// std::chrono::duration from toml::local_time
+
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_chrono_duration<T>::value, T>
+get(const basic_value<TC>& v)
+{
+    return std::chrono::duration_cast<T>(
+            std::chrono::nanoseconds(v.as_local_time()));
+}
+
+// ============================================================================
+// std::chrono::system_clock::time_point from toml::datetime variants
+
+template<typename T, typename TC>
+cxx::enable_if_t<
+    std::is_same<std::chrono::system_clock::time_point, T>::value, T>
+get(const basic_value<TC>& v)
+{
+    switch(v.type())
+    {
+        case value_t::local_date:
+        {
+            return std::chrono::system_clock::time_point(v.as_local_date());
+        }
+        case value_t::local_datetime:
+        {
+            return std::chrono::system_clock::time_point(v.as_local_datetime());
+        }
+        case value_t::offset_datetime:
+        {
+            return std::chrono::system_clock::time_point(v.as_offset_datetime());
+        }
+        default:
+        {
+            const auto loc = v.location();
+            throw type_error(format_error("toml::get: "
+                "bad_cast to std::chrono::system_clock::time_point", loc,
+                "the actual type is " + to_string(v.type())), loc);
+        }
+    }
+}
+
+// ============================================================================
+// forward declaration to use this recursively. ignore this and go ahead.
+
+// array-like (w/ push_back)
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_container<T>,                            // T is a container
+    detail::has_push_back_method<T>,                    // .push_back() works
+    detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::array
+    cxx::negation<detail::is_std_basic_string<T>>,      // but not std::basic_string<CharT>
+#if defined(TOML11_HAS_STRING_VIEW)
+    cxx::negation<detail::is_std_basic_string_view<T>>,      // but not std::basic_string_view<CharT>
+#endif
+    cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
+    cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
+    >::value, T>
+get(const basic_value<TC>&);
+
+// std::array
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_std_array<T>::value, T>
+get(const basic_value<TC>&);
+
+// std::forward_list
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_std_forward_list<T>::value, T>
+get(const basic_value<TC>&);
+
+// std::pair<T1, T2>
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_std_pair<T>::value, T>
+get(const basic_value<TC>&);
+
+// std::tuple<T1, T2, ...>
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_std_tuple<T>::value, T>
+get(const basic_value<TC>&);
+
+// std::map<key, value> (key is convertible from toml::value::key_type)
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_map<T>,                                  // T is map
+    detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::table
+    std::is_convertible<typename basic_value<TC>::key_type,
+                        typename T::key_type>,          // keys are convertible
+    cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
+    cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
+    >::value, T>
+get(const basic_value<TC>& v);
+
+// std::map<key, value> (key is not convertible from toml::value::key_type, but
+// is a std::basic_string)
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_map<T>,                                  // T is map
+    detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::table
+    cxx::negation<std::is_convertible<typename basic_value<TC>::key_type,
+        typename T::key_type>>,                         // keys are NOT convertible
+    detail::is_1byte_std_basic_string<typename T::key_type>, // is std::basic_string
+    cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
+    cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
+    >::value, T>
+get(const basic_value<TC>& v);
+
+// toml::from<T>::from_toml(v)
+template<typename T, typename TC>
+cxx::enable_if_t<detail::has_specialized_from<T>::value, T>
+get(const basic_value<TC>&);
+
+// has T.from_toml(v) but no from<T>
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::has_from_toml_method<T, TC>,            // has T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>, // no toml::from<T>
+    std::is_default_constructible<T>                // T{} works
+    >::value, T>
+get(const basic_value<TC>&);
+
+// T(const toml::value&) and T is not toml::basic_value,
+// and it does not have `from<T>` nor `from_toml`.
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    std::is_constructible<T, const basic_value<TC>&>,   // has T(const basic_value&)
+    cxx::negation<detail::is_basic_value<T>>,           // but not basic_value itself
+    cxx::negation<detail::has_from_toml_method<T, TC>>, // no .from_toml()
+    cxx::negation<detail::has_specialized_from<T>>      // no toml::from<T>
+    >::value, T>
+get(const basic_value<TC>&);
+
+// ============================================================================
+// array-like types; most likely STL container, like std::vector, etc.
+
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_container<T>,                            // T is a container
+    detail::has_push_back_method<T>,                    // .push_back() works
+    detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::array
+    cxx::negation<detail::is_std_basic_string<T>>,      // but not std::basic_string<CharT>
+#if defined(TOML11_HAS_STRING_VIEW)
+    cxx::negation<detail::is_std_basic_string_view<T>>, // but not std::basic_string_view<CharT>
+#endif
+    cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
+    cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    using value_type = typename T::value_type;
+    const auto& a = v.as_array();
+
+    T container;
+    detail::try_reserve(container, a.size()); // if T has .reserve(), call it
+
+    for(const auto& elem : a)
+    {
+        container.push_back(get<value_type>(elem));
+    }
+    return container;
+}
+
+// ============================================================================
+// std::array
+
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_std_array<T>::value, T>
+get(const basic_value<TC>& v)
+{
+    using value_type = typename T::value_type;
+    const auto& a = v.as_array();
+
+    T container;
+    if(a.size() != container.size())
+    {
+        const auto loc = v.location();
+        throw std::out_of_range(format_error("toml::get: while converting to an array: "
+            " array size is " + std::to_string(container.size()) +
+            " but there are " + std::to_string(a.size()) + " elements in toml array.",
+            loc, "here"));
+    }
+    for(std::size_t i=0; i<a.size(); ++i)
+    {
+        container.at(i) = ::toml::get<value_type>(a.at(i));
+    }
+    return container;
+}
+
+// ============================================================================
+// std::forward_list
+
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_std_forward_list<T>::value, T>
+get(const basic_value<TC>& v)
+{
+    using value_type = typename T::value_type;
+
+    T container;
+    for(const auto& elem : v.as_array())
+    {
+        container.push_front(get<value_type>(elem));
+    }
+    container.reverse();
+    return container;
+}
+
+// ============================================================================
+// std::pair
+
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_std_pair<T>::value, T>
+get(const basic_value<TC>& v)
+{
+    using first_type  = typename T::first_type;
+    using second_type = typename T::second_type;
+
+    const auto& ar = v.as_array();
+    if(ar.size() != 2)
+    {
+        const auto loc = v.location();
+        throw std::out_of_range(format_error("toml::get: while converting std::pair: "
+            " but there are " + std::to_string(ar.size()) + " > 2 elements in toml array.",
+            loc, "here"));
+    }
+    return std::make_pair(::toml::get<first_type >(ar.at(0)),
+                          ::toml::get<second_type>(ar.at(1)));
+}
+
+// ============================================================================
+// std::tuple.
+
+namespace detail
+{
+template<typename T, typename Array, std::size_t ... I>
+T get_tuple_impl(const Array& a, cxx::index_sequence<I...>)
+{
+    return std::make_tuple(
+        ::toml::get<typename std::tuple_element<I, T>::type>(a.at(I))...);
+}
+} // detail
+
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_std_tuple<T>::value, T>
+get(const basic_value<TC>& v)
+{
+    const auto& ar = v.as_array();
+    if(ar.size() != std::tuple_size<T>::value)
+    {
+        const auto loc = v.location();
+        throw std::out_of_range(format_error("toml::get: while converting std::tuple: "
+            " there are " + std::to_string(ar.size()) + " > " +
+            std::to_string(std::tuple_size<T>::value) + " elements in toml array.",
+            loc, "here"));
+    }
+    return detail::get_tuple_impl<T>(ar,
+            cxx::make_index_sequence<std::tuple_size<T>::value>{});
+}
+
+// ============================================================================
+// map-like types; most likely STL map, like std::map or std::unordered_map.
+
+// key is convertible from toml::value::key_type
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_map<T>,                                  // T is map
+    detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::table
+    std::is_convertible<typename basic_value<TC>::key_type,
+                        typename T::key_type>,          // keys are convertible
+    cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
+    cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    using key_type    = typename T::key_type;
+    using mapped_type = typename T::mapped_type;
+    static_assert(
+        std::is_convertible<typename basic_value<TC>::key_type, key_type>::value,
+        "toml::get only supports map type of which key_type is "
+        "convertible from toml::basic_value::key_type.");
+
+    T m;
+    for(const auto& kv : v.as_table())
+    {
+        m.emplace(key_type(kv.first), get<mapped_type>(kv.second));
+    }
+    return m;
+}
+
+// key is NOT convertible from toml::value::key_type but std::basic_string
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_map<T>,                                       // T is map
+    detail::is_not_toml_type<T, basic_value<TC>>,            // but not toml::table
+    cxx::negation<std::is_convertible<typename basic_value<TC>::key_type,
+        typename T::key_type>>,                              // keys are NOT convertible
+    detail::is_1byte_std_basic_string<typename T::key_type>, // is std::basic_string
+    cxx::negation<detail::has_from_toml_method<T, TC>>,      // no T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>,          // no toml::from<T>
+    cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    using key_type    = typename T::key_type;
+    using mapped_type = typename T::mapped_type;
+
+    T m;
+    for(const auto& kv : v.as_table())
+    {
+        m.emplace(detail::string_conv<key_type>(kv.first), get<mapped_type>(kv.second));
+    }
+    return m;
+}
+
+// ============================================================================
+// user-defined, but convertible types.
+
+// toml::from<T>
+template<typename T, typename TC>
+cxx::enable_if_t<detail::has_specialized_from<T>::value, T>
+get(const basic_value<TC>& v)
+{
+    return ::toml::from<T>::from_toml(v);
+}
+
+// has T.from_toml(v) but no from<T>
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::has_from_toml_method<T, TC>,            // has T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>, // no toml::from<T>
+    std::is_default_constructible<T>                // T{} works
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    T ud;
+    ud.from_toml(v);
+    return ud;
+}
+
+// T(const toml::value&) and T is not toml::basic_value,
+// and it does not have `from<T>` nor `from_toml`.
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    std::is_constructible<T, const basic_value<TC>&>,   // has T(const basic_value&)
+    cxx::negation<detail::is_basic_value<T>>,           // but not basic_value itself
+    cxx::negation<detail::has_from_toml_method<T, TC>>, // no .from_toml()
+    cxx::negation<detail::has_specialized_from<T>>      // no toml::from<T>
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    return T(v);
+}
+
+// ============================================================================
+// get_or(value, fallback)
+
+template<typename TC>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>> const&
+get_or(const basic_value<TC>& v, const basic_value<TC>&)
+{
+    return v;
+}
+
+template<typename TC>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>&
+get_or(basic_value<TC>& v, basic_value<TC>&)
+{
+    return v;
+}
+
+template<typename TC>
+cxx::enable_if_t<detail::is_type_config<TC>::value, basic_value<TC>>
+get_or(basic_value<TC>&& v, basic_value<TC>&&)
+{
+    return v;
+}
+
+// ----------------------------------------------------------------------------
+// specialization for the exact toml types (return type becomes lvalue ref)
+
+template<typename T, typename TC>
+cxx::enable_if_t<
+    detail::is_exact_toml_type<T, basic_value<TC>>::value, T> const&
+get_or(const basic_value<TC>& v, const T& opt) noexcept
+{
+    try
+    {
+        return get<cxx::remove_cvref_t<T>>(v);
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+        cxx::negation<std::is_const<T>>,
+        detail::is_exact_toml_type<T, basic_value<TC>>
+    >::value, T>&
+get_or(basic_value<TC>& v, T& opt) noexcept
+{
+    try
+    {
+        return get<cxx::remove_cvref_t<T>>(v);
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+template<typename T, typename TC>
+cxx::enable_if_t<detail::is_exact_toml_type<cxx::remove_cvref_t<T>,
+    basic_value<TC>>::value, cxx::remove_cvref_t<T>>
+get_or(basic_value<TC>&& v, T&& opt) noexcept
+{
+    try
+    {
+        return get<cxx::remove_cvref_t<T>>(std::move(v));
+    }
+    catch(...)
+    {
+        return cxx::remove_cvref_t<T>(std::forward<T>(opt));
+    }
+}
+
+// ----------------------------------------------------------------------------
+// specialization for string literal
+
+// template<std::size_t N, typename TC>
+// typename basic_value<TC>::string_type
+// get_or(const basic_value<TC>& v,
+//        const typename basic_value<TC>::string_type::value_type (&opt)[N])
+// {
+//     try
+//     {
+//         return v.as_string();
+//     }
+//     catch(...)
+//     {
+//         return typename basic_value<TC>::string_type(opt);
+//     }
+// }
+//
+// The above only matches to the literal, like `get_or(v, "foo");` but not
+// ```cpp
+// const auto opt = "foo";
+// const auto str = get_or(v, opt);
+// ```
+// . And the latter causes an error.
+// To match to both `"foo"` and `const auto opt = "foo"`, we take a pointer to
+// a character here.
+
+template<typename TC>
+typename basic_value<TC>::string_type
+get_or(const basic_value<TC>& v,
+       const typename basic_value<TC>::string_type::value_type* opt)
+{
+    try
+    {
+        return v.as_string();
+    }
+    catch(...)
+    {
+        return typename basic_value<TC>::string_type(opt);
+    }
+}
+
+// ----------------------------------------------------------------------------
+// others (require type conversion and return type cannot be lvalue reference)
+
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    cxx::negation<detail::is_basic_value<T>>,
+    cxx::negation<detail::is_exact_toml_type<T, basic_value<TC>>>,
+    cxx::negation<std::is_same<cxx::remove_cvref_t<T>, typename basic_value<TC>::string_type::value_type const*>>
+    >::value, cxx::remove_cvref_t<T>>
+get_or(const basic_value<TC>& v, T&& opt)
+{
+    try
+    {
+        return get<cxx::remove_cvref_t<T>>(v);
+    }
+    catch(...)
+    {
+        return cxx::remove_cvref_t<T>(std::forward<T>(opt));
+    }
+}
+
+} // toml
+#endif // TOML11_GET_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/color_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/color_impl.hpp
new file mode 100644
index 00000000..4215587f
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/color_impl.hpp
@@ -0,0 +1,76 @@
+#ifndef TOML11_COLOR_IMPL_HPP
+#define TOML11_COLOR_IMPL_HPP
+
+#include "../fwd/color_fwd.hpp"
+#include "../version.hpp"
+
+#include <ostream>
+
+namespace toml
+{
+namespace color
+{
+// put ANSI escape sequence to ostream
+inline namespace ansi
+{
+
+TOML11_INLINE std::ostream& reset(std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[00m";}
+    return os;
+}
+TOML11_INLINE std::ostream& bold(std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[01m";}
+    return os;
+}
+TOML11_INLINE std::ostream& grey(std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[30m";}
+    return os;
+}
+TOML11_INLINE std::ostream& gray(std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[30m";}
+    return os;
+}
+TOML11_INLINE std::ostream& red(std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[31m";}
+    return os;
+}
+TOML11_INLINE std::ostream& green(std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[32m";}
+    return os;
+}
+TOML11_INLINE std::ostream& yellow(std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[33m";}
+    return os;
+}
+TOML11_INLINE std::ostream& blue(std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[34m";}
+    return os;
+}
+TOML11_INLINE std::ostream& magenta(std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[35m";}
+    return os;
+}
+TOML11_INLINE std::ostream& cyan   (std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[36m";}
+    return os;
+}
+TOML11_INLINE std::ostream& white  (std::ostream& os)
+{
+    if(detail::color_status().should_color()) {os << "\033[37m";}
+    return os;
+}
+
+} // ansi
+} // color
+} // toml
+#endif // TOML11_COLOR_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/comments_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/comments_impl.hpp
new file mode 100644
index 00000000..25023eba
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/comments_impl.hpp
@@ -0,0 +1,46 @@
+#ifndef TOML11_COMMENTS_IMPL_HPP
+#define TOML11_COMMENTS_IMPL_HPP
+
+#include "../fwd/comments_fwd.hpp" // IWYU pragma: keep
+
+namespace toml
+{
+
+TOML11_INLINE bool operator==(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments == rhs.comments;}
+TOML11_INLINE bool operator!=(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments != rhs.comments;}
+TOML11_INLINE bool operator< (const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments <  rhs.comments;}
+TOML11_INLINE bool operator<=(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments <= rhs.comments;}
+TOML11_INLINE bool operator> (const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments >  rhs.comments;}
+TOML11_INLINE bool operator>=(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments >= rhs.comments;}
+
+TOML11_INLINE void swap(preserve_comments& lhs, preserve_comments& rhs)
+{
+    lhs.swap(rhs);
+    return;
+}
+TOML11_INLINE void swap(preserve_comments& lhs, std::vector<std::string>& rhs)
+{
+    lhs.comments.swap(rhs);
+    return;
+}
+TOML11_INLINE void swap(std::vector<std::string>& lhs, preserve_comments& rhs)
+{
+    lhs.swap(rhs.comments);
+    return;
+}
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const preserve_comments& com)
+{
+    for(const auto& c : com)
+    {
+        if(c.front() != '#')
+        {
+            os << '#';
+        }
+        os << c << '\n';
+    }
+    return os;
+}
+
+} // toml11
+#endif // TOML11_COMMENTS_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/datetime_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/datetime_impl.hpp
new file mode 100644
index 00000000..50c04557
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/datetime_impl.hpp
@@ -0,0 +1,518 @@
+#ifndef TOML11_DATETIME_IMPL_HPP
+#define TOML11_DATETIME_IMPL_HPP
+
+#include "../fwd/datetime_fwd.hpp"
+#include "../version.hpp"
+
+#include <array>
+#include <iomanip>
+#include <ostream>
+#include <sstream>
+#include <tuple>
+
+#include <cstdlib>
+#include <ctime>
+
+namespace toml
+{
+
+// To avoid non-threadsafe std::localtime. In C11 (not C++11!), localtime_s is
+// provided in the absolutely same purpose, but C++11 is actually not compatible
+// with C11. We need to dispatch the function depending on the OS.
+namespace detail
+{
+// TODO: find more sophisticated way to handle this
+#if defined(_MSC_VER)
+TOML11_INLINE std::tm localtime_s(const std::time_t* src)
+{
+    std::tm dst;
+    const auto result = ::localtime_s(&dst, src);
+    if (result) { throw std::runtime_error("localtime_s failed."); }
+    return dst;
+}
+TOML11_INLINE std::tm gmtime_s(const std::time_t* src)
+{
+    std::tm dst;
+    const auto result = ::gmtime_s(&dst, src);
+    if (result) { throw std::runtime_error("gmtime_s failed."); }
+    return dst;
+}
+#elif (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 1) || defined(_XOPEN_SOURCE) || defined(_BSD_SOURCE) || defined(_SVID_SOURCE) || defined(_POSIX_SOURCE)
+TOML11_INLINE std::tm localtime_s(const std::time_t* src)
+{
+    std::tm dst;
+    const auto result = ::localtime_r(src, &dst);
+    if (!result) { throw std::runtime_error("localtime_r failed."); }
+    return dst;
+}
+TOML11_INLINE std::tm gmtime_s(const std::time_t* src)
+{
+    std::tm dst;
+    const auto result = ::gmtime_r(src, &dst);
+    if (!result) { throw std::runtime_error("gmtime_r failed."); }
+    return dst;
+}
+#else // fallback. not threadsafe
+TOML11_INLINE std::tm localtime_s(const std::time_t* src)
+{
+    const auto result = std::localtime(src);
+    if (!result) { throw std::runtime_error("localtime failed."); }
+    return *result;
+}
+TOML11_INLINE std::tm gmtime_s(const std::time_t* src)
+{
+    const auto result = std::gmtime(src);
+    if (!result) { throw std::runtime_error("gmtime failed."); }
+    return *result;
+}
+#endif
+} // detail
+
+// ----------------------------------------------------------------------------
+
+TOML11_INLINE local_date::local_date(const std::chrono::system_clock::time_point& tp)
+{
+    const auto t    = std::chrono::system_clock::to_time_t(tp);
+    const auto time = detail::localtime_s(&t);
+    *this = local_date(time);
+}
+
+TOML11_INLINE local_date::local_date(const std::time_t t)
+    : local_date{std::chrono::system_clock::from_time_t(t)}
+{}
+
+TOML11_INLINE local_date::operator std::chrono::system_clock::time_point() const
+{
+    // std::mktime returns date as local time zone. no conversion needed
+    std::tm t;
+    t.tm_sec   = 0;
+    t.tm_min   = 0;
+    t.tm_hour  = 0;
+    t.tm_mday  = static_cast<int>(this->day);
+    t.tm_mon   = static_cast<int>(this->month);
+    t.tm_year  = static_cast<int>(this->year) - 1900;
+    t.tm_wday  = 0; // the value will be ignored
+    t.tm_yday  = 0; // the value will be ignored
+    t.tm_isdst = -1;
+    return std::chrono::system_clock::from_time_t(std::mktime(&t));
+}
+
+TOML11_INLINE local_date::operator std::time_t() const
+{
+    return std::chrono::system_clock::to_time_t(
+            std::chrono::system_clock::time_point(*this));
+}
+
+TOML11_INLINE bool operator==(const local_date& lhs, const local_date& rhs)
+{
+    return std::make_tuple(lhs.year, lhs.month, lhs.day) ==
+           std::make_tuple(rhs.year, rhs.month, rhs.day);
+}
+TOML11_INLINE bool operator!=(const local_date& lhs, const local_date& rhs)
+{
+    return !(lhs == rhs);
+}
+TOML11_INLINE bool operator< (const local_date& lhs, const local_date& rhs)
+{
+    return std::make_tuple(lhs.year, lhs.month, lhs.day) <
+           std::make_tuple(rhs.year, rhs.month, rhs.day);
+}
+TOML11_INLINE bool operator<=(const local_date& lhs, const local_date& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+TOML11_INLINE bool operator> (const local_date& lhs, const local_date& rhs)
+{
+    return !(lhs <= rhs);
+}
+TOML11_INLINE bool operator>=(const local_date& lhs, const local_date& rhs)
+{
+    return !(lhs < rhs);
+}
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const local_date& date)
+{
+    os << std::setfill('0') << std::setw(4) << static_cast<int>(date.year )     << '-';
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(date.month) + 1 << '-';
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(date.day  )    ;
+    return os;
+}
+
+TOML11_INLINE std::string to_string(const local_date& date)
+{
+    std::ostringstream oss;
+    oss.imbue(std::locale::classic());
+    oss << date;
+    return oss.str();
+}
+
+// -----------------------------------------------------------------------------
+
+TOML11_INLINE local_time::operator std::chrono::nanoseconds() const
+{
+    return std::chrono::nanoseconds (this->nanosecond)  +
+           std::chrono::microseconds(this->microsecond) +
+           std::chrono::milliseconds(this->millisecond) +
+           std::chrono::seconds(this->second) +
+           std::chrono::minutes(this->minute) +
+           std::chrono::hours(this->hour);
+}
+
+TOML11_INLINE bool operator==(const local_time& lhs, const local_time& rhs)
+{
+    return std::make_tuple(lhs.hour, lhs.minute, lhs.second, lhs.millisecond, lhs.microsecond, lhs.nanosecond) ==
+           std::make_tuple(rhs.hour, rhs.minute, rhs.second, rhs.millisecond, rhs.microsecond, rhs.nanosecond);
+}
+TOML11_INLINE bool operator!=(const local_time& lhs, const local_time& rhs)
+{
+    return !(lhs == rhs);
+}
+TOML11_INLINE bool operator< (const local_time& lhs, const local_time& rhs)
+{
+    return std::make_tuple(lhs.hour, lhs.minute, lhs.second, lhs.millisecond, lhs.microsecond, lhs.nanosecond) <
+           std::make_tuple(rhs.hour, rhs.minute, rhs.second, rhs.millisecond, rhs.microsecond, rhs.nanosecond);
+}
+TOML11_INLINE bool operator<=(const local_time& lhs, const local_time& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+TOML11_INLINE bool operator> (const local_time& lhs, const local_time& rhs)
+{
+    return !(lhs <= rhs);
+}
+TOML11_INLINE bool operator>=(const local_time& lhs, const local_time& rhs)
+{
+    return !(lhs < rhs);
+}
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const local_time& time)
+{
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(time.hour  ) << ':';
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(time.minute) << ':';
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(time.second);
+    if(time.millisecond != 0 || time.microsecond != 0 || time.nanosecond != 0)
+    {
+        os << '.';
+        os << std::setfill('0') << std::setw(3) << static_cast<int>(time.millisecond);
+        if(time.microsecond != 0 || time.nanosecond != 0)
+        {
+            os << std::setfill('0') << std::setw(3) << static_cast<int>(time.microsecond);
+            if(time.nanosecond != 0)
+            {
+                os << std::setfill('0') << std::setw(3) << static_cast<int>(time.nanosecond);
+            }
+        }
+    }
+    return os;
+}
+
+TOML11_INLINE std::string to_string(const local_time& time)
+{
+    std::ostringstream oss;
+    oss.imbue(std::locale::classic());
+    oss << time;
+    return oss.str();
+}
+
+// ----------------------------------------------------------------------------
+
+TOML11_INLINE time_offset::operator std::chrono::minutes() const
+{
+    return std::chrono::minutes(this->minute) +
+           std::chrono::hours(this->hour);
+}
+
+TOML11_INLINE bool operator==(const time_offset& lhs, const time_offset& rhs)
+{
+    return std::make_tuple(lhs.hour, lhs.minute) ==
+           std::make_tuple(rhs.hour, rhs.minute);
+}
+TOML11_INLINE bool operator!=(const time_offset& lhs, const time_offset& rhs)
+{
+    return !(lhs == rhs);
+}
+TOML11_INLINE bool operator< (const time_offset& lhs, const time_offset& rhs)
+{
+    return std::make_tuple(lhs.hour, lhs.minute) <
+           std::make_tuple(rhs.hour, rhs.minute);
+}
+TOML11_INLINE bool operator<=(const time_offset& lhs, const time_offset& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+TOML11_INLINE bool operator> (const time_offset& lhs, const time_offset& rhs)
+{
+    return !(lhs <= rhs);
+}
+TOML11_INLINE bool operator>=(const time_offset& lhs, const time_offset& rhs)
+{
+    return !(lhs < rhs);
+}
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const time_offset& offset)
+{
+    if(offset.hour == 0 && offset.minute == 0)
+    {
+        os << 'Z';
+        return os;
+    }
+    int minute = static_cast<int>(offset.hour) * 60 + offset.minute;
+    if(minute < 0){os << '-'; minute = std::abs(minute);} else {os << '+';}
+    os << std::setfill('0') << std::setw(2) << minute / 60 << ':';
+    os << std::setfill('0') << std::setw(2) << minute % 60;
+    return os;
+}
+
+TOML11_INLINE std::string to_string(const time_offset& offset)
+{
+    std::ostringstream oss;
+    oss.imbue(std::locale::classic());
+    oss << offset;
+    return oss.str();
+}
+
+// -----------------------------------------------------------------------------
+
+TOML11_INLINE local_datetime::local_datetime(const std::chrono::system_clock::time_point& tp)
+{
+    const auto t = std::chrono::system_clock::to_time_t(tp);
+    std::tm ltime = detail::localtime_s(&t);
+
+    this->date = local_date(ltime);
+    this->time = local_time(ltime);
+
+    // std::tm lacks subsecond information, so diff between tp and tm
+    // can be used to get millisecond & microsecond information.
+    const auto t_diff = tp -
+        std::chrono::system_clock::from_time_t(std::mktime(&ltime));
+    this->time.millisecond = static_cast<std::uint16_t>(
+      std::chrono::duration_cast<std::chrono::milliseconds>(t_diff).count());
+    this->time.microsecond = static_cast<std::uint16_t>(
+      std::chrono::duration_cast<std::chrono::microseconds>(t_diff).count());
+    this->time.nanosecond = static_cast<std::uint16_t>(
+      std::chrono::duration_cast<std::chrono::nanoseconds >(t_diff).count());
+}
+
+TOML11_INLINE local_datetime::local_datetime(const std::time_t t)
+    : local_datetime{std::chrono::system_clock::from_time_t(t)}
+{}
+
+TOML11_INLINE local_datetime::operator std::chrono::system_clock::time_point() const
+{
+    using internal_duration =
+        typename std::chrono::system_clock::time_point::duration;
+
+    // Normally DST begins at A.M. 3 or 4. If we re-use conversion operator
+    // of local_date and local_time independently, the conversion fails if
+    // it is the day when DST begins or ends. Since local_date considers the
+    // time is 00:00 A.M. and local_time does not consider DST because it
+    // does not have any date information. We need to consider both date and
+    // time information at the same time to convert it correctly.
+
+    std::tm t;
+    t.tm_sec   = static_cast<int>(this->time.second);
+    t.tm_min   = static_cast<int>(this->time.minute);
+    t.tm_hour  = static_cast<int>(this->time.hour);
+    t.tm_mday  = static_cast<int>(this->date.day);
+    t.tm_mon   = static_cast<int>(this->date.month);
+    t.tm_year  = static_cast<int>(this->date.year) - 1900;
+    t.tm_wday  = 0; // the value will be ignored
+    t.tm_yday  = 0; // the value will be ignored
+    t.tm_isdst = -1;
+
+    // std::mktime returns date as local time zone. no conversion needed
+    auto dt = std::chrono::system_clock::from_time_t(std::mktime(&t));
+    dt += std::chrono::duration_cast<internal_duration>(
+            std::chrono::milliseconds(this->time.millisecond) +
+            std::chrono::microseconds(this->time.microsecond) +
+            std::chrono::nanoseconds (this->time.nanosecond));
+    return dt;
+}
+
+TOML11_INLINE local_datetime::operator std::time_t() const
+{
+    return std::chrono::system_clock::to_time_t(
+            std::chrono::system_clock::time_point(*this));
+}
+
+TOML11_INLINE bool operator==(const local_datetime& lhs, const local_datetime& rhs)
+{
+    return std::make_tuple(lhs.date, lhs.time) ==
+           std::make_tuple(rhs.date, rhs.time);
+}
+TOML11_INLINE bool operator!=(const local_datetime& lhs, const local_datetime& rhs)
+{
+    return !(lhs == rhs);
+}
+TOML11_INLINE bool operator< (const local_datetime& lhs, const local_datetime& rhs)
+{
+    return std::make_tuple(lhs.date, lhs.time) <
+           std::make_tuple(rhs.date, rhs.time);
+}
+TOML11_INLINE bool operator<=(const local_datetime& lhs, const local_datetime& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+TOML11_INLINE bool operator> (const local_datetime& lhs, const local_datetime& rhs)
+{
+    return !(lhs <= rhs);
+}
+TOML11_INLINE bool operator>=(const local_datetime& lhs, const local_datetime& rhs)
+{
+    return !(lhs < rhs);
+}
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const local_datetime& dt)
+{
+    os << dt.date << 'T' << dt.time;
+    return os;
+}
+
+TOML11_INLINE std::string to_string(const local_datetime& dt)
+{
+    std::ostringstream oss;
+    oss.imbue(std::locale::classic());
+    oss << dt;
+    return oss.str();
+}
+
+// -----------------------------------------------------------------------------
+
+
+TOML11_INLINE offset_datetime::offset_datetime(const local_datetime& ld)
+    : date{ld.date}, time{ld.time}, offset{get_local_offset(nullptr)}
+      // use the current local timezone offset
+{}
+TOML11_INLINE offset_datetime::offset_datetime(const std::chrono::system_clock::time_point& tp)
+    : offset{0, 0} // use gmtime
+{
+    const auto timet = std::chrono::system_clock::to_time_t(tp);
+    const auto tm    = detail::gmtime_s(&timet);
+    this->date = local_date(tm);
+    this->time = local_time(tm);
+}
+TOML11_INLINE offset_datetime::offset_datetime(const std::time_t& t)
+    : offset{0, 0} // use gmtime
+{
+    const auto tm    = detail::gmtime_s(&t);
+    this->date = local_date(tm);
+    this->time = local_time(tm);
+}
+TOML11_INLINE offset_datetime::offset_datetime(const std::tm& t)
+    : offset{0, 0} // assume gmtime
+{
+    this->date = local_date(t);
+    this->time = local_time(t);
+}
+
+TOML11_INLINE offset_datetime::operator std::chrono::system_clock::time_point() const
+{
+    // get date-time
+    using internal_duration =
+        typename std::chrono::system_clock::time_point::duration;
+
+    // first, convert it to local date-time information in the same way as
+    // local_datetime does. later we will use time_t to adjust time offset.
+    std::tm t;
+    t.tm_sec   = static_cast<int>(this->time.second);
+    t.tm_min   = static_cast<int>(this->time.minute);
+    t.tm_hour  = static_cast<int>(this->time.hour);
+    t.tm_mday  = static_cast<int>(this->date.day);
+    t.tm_mon   = static_cast<int>(this->date.month);
+    t.tm_year  = static_cast<int>(this->date.year) - 1900;
+    t.tm_wday  = 0; // the value will be ignored
+    t.tm_yday  = 0; // the value will be ignored
+    t.tm_isdst = -1;
+    const std::time_t tp_loc = std::mktime(std::addressof(t));
+
+    auto tp = std::chrono::system_clock::from_time_t(tp_loc);
+    tp += std::chrono::duration_cast<internal_duration>(
+            std::chrono::milliseconds(this->time.millisecond) +
+            std::chrono::microseconds(this->time.microsecond) +
+            std::chrono::nanoseconds (this->time.nanosecond));
+
+    // Since mktime uses local time zone, it should be corrected.
+    // `12:00:00+09:00` means `03:00:00Z`. So mktime returns `03:00:00Z` if
+    // we are in `+09:00` timezone. To represent `12:00:00Z` there, we need
+    // to add `+09:00` to `03:00:00Z`.
+    //    Here, it uses the time_t converted from date-time info to handle
+    // daylight saving time.
+    const auto ofs = get_local_offset(std::addressof(tp_loc));
+    tp += std::chrono::hours  (ofs.hour);
+    tp += std::chrono::minutes(ofs.minute);
+
+    // We got `12:00:00Z` by correcting local timezone applied by mktime.
+    // Then we will apply the offset. Let's say `12:00:00-08:00` is given.
+    // And now, we have `12:00:00Z`. `12:00:00-08:00` means `20:00:00Z`.
+    // So we need to subtract the offset.
+    tp -= std::chrono::minutes(this->offset);
+    return tp;
+}
+
+TOML11_INLINE offset_datetime::operator std::time_t() const
+{
+    return std::chrono::system_clock::to_time_t(
+            std::chrono::system_clock::time_point(*this));
+}
+
+TOML11_INLINE time_offset offset_datetime::get_local_offset(const std::time_t* tp)
+{
+    // get local timezone with the same date-time information as mktime
+    const auto t = detail::localtime_s(tp);
+
+    std::array<char, 6> buf;
+    const auto result = std::strftime(buf.data(), 6, "%z", &t); // +hhmm\0
+    if(result != 5)
+    {
+        throw std::runtime_error("toml::offset_datetime: cannot obtain "
+                                 "timezone information of current env");
+    }
+    const int ofs = std::atoi(buf.data());
+    const int ofs_h = ofs / 100;
+    const int ofs_m = ofs - (ofs_h * 100);
+    return time_offset(ofs_h, ofs_m);
+}
+
+TOML11_INLINE bool operator==(const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return std::make_tuple(lhs.date, lhs.time, lhs.offset) ==
+           std::make_tuple(rhs.date, rhs.time, rhs.offset);
+}
+TOML11_INLINE bool operator!=(const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return !(lhs == rhs);
+}
+TOML11_INLINE bool operator< (const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return std::make_tuple(lhs.date, lhs.time, lhs.offset) <
+           std::make_tuple(rhs.date, rhs.time, rhs.offset);
+}
+TOML11_INLINE bool operator<=(const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+TOML11_INLINE bool operator> (const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return !(lhs <= rhs);
+}
+TOML11_INLINE bool operator>=(const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return !(lhs < rhs);
+}
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const offset_datetime& dt)
+{
+    os << dt.date << 'T' << dt.time << dt.offset;
+    return os;
+}
+
+TOML11_INLINE std::string to_string(const offset_datetime& dt)
+{
+    std::ostringstream oss;
+    oss.imbue(std::locale::classic());
+    oss << dt;
+    return oss.str();
+}
+
+}//toml
+#endif // TOML11_DATETIME_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/error_info_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/error_info_impl.hpp
new file mode 100644
index 00000000..b87f4776
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/error_info_impl.hpp
@@ -0,0 +1,75 @@
+#ifndef TOML11_ERROR_INFO_IMPL_HPP
+#define TOML11_ERROR_INFO_IMPL_HPP
+
+#include "../fwd/error_info_fwd.hpp"
+#include "../fwd/color_fwd.hpp"
+
+#include <sstream>
+
+namespace toml
+{
+
+TOML11_INLINE std::string format_error(const std::string& errkind, const error_info& err)
+{
+    std::string errmsg;
+    if( ! errkind.empty())
+    {
+        errmsg = errkind;
+        errmsg += ' ';
+    }
+    errmsg += err.title();
+    errmsg += '\n';
+
+    const auto lnw = [&err]() {
+        std::size_t width = 0;
+        for(const auto& l : err.locations())
+        {
+            width = (std::max)(detail::integer_width_base10(l.first.last_line_number()), width);
+        }
+        return width;
+    }();
+
+    bool first = true;
+    std::string prev_fname;
+    for(const auto& lm : err.locations())
+    {
+        if( ! first)
+        {
+            std::ostringstream oss;
+            oss << detail::make_string(lnw + 1, ' ')
+                << color::bold << color::blue << " |" << color::reset
+                << color::bold << " ...\n" << color::reset;
+            oss << detail::make_string(lnw + 1, ' ')
+                << color::bold << color::blue << " |\n" << color::reset;
+            errmsg += oss.str();
+        }
+
+        const auto& l = lm.first;
+        const auto& m = lm.second;
+
+        errmsg += detail::format_location_impl(lnw, prev_fname, l, m);
+
+        prev_fname = l.file_name();
+        first = false;
+    }
+
+    errmsg += err.suffix();
+
+    return errmsg;
+}
+
+TOML11_INLINE std::string format_error(const error_info& err)
+{
+    std::ostringstream oss;
+    oss << color::red << color::bold << "[error]" << color::reset;
+    return format_error(oss.str(), err);
+}
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const error_info& e)
+{
+    os << format_error(e);
+    return os;
+}
+
+} // toml
+#endif // TOML11_ERROR_INFO_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/format_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/format_impl.hpp
new file mode 100644
index 00000000..c4985faf
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/format_impl.hpp
@@ -0,0 +1,297 @@
+#ifndef TOML11_FORMAT_IMPL_HPP
+#define TOML11_FORMAT_IMPL_HPP
+
+#include "../fwd/format_fwd.hpp"
+#include "../version.hpp"
+
+#include <ostream>
+#include <sstream>
+
+namespace toml
+{
+
+// toml types with serialization info
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const indent_char& c)
+{
+    switch(c)
+    {
+        case indent_char::space:         {os << "space"        ; break;}
+        case indent_char::tab:           {os << "tab"          ; break;}
+        case indent_char::none:          {os << "none"         ; break;}
+        default:
+        {
+            os << "unknown indent char: " << static_cast<std::uint8_t>(c);
+        }
+    }
+    return os;
+}
+
+TOML11_INLINE std::string to_string(const indent_char c)
+{
+    std::ostringstream oss;
+    oss << c;
+    return oss.str();
+}
+
+// ----------------------------------------------------------------------------
+// boolean
+
+// ----------------------------------------------------------------------------
+// integer
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const integer_format f)
+{
+    switch(f)
+    {
+        case integer_format::dec: {os << "dec"; break;}
+        case integer_format::bin: {os << "bin"; break;}
+        case integer_format::oct: {os << "oct"; break;}
+        case integer_format::hex: {os << "hex"; break;}
+        default:
+        {
+            os << "unknown integer_format: " << static_cast<std::uint8_t>(f);
+            break;
+        }
+    }
+    return os;
+}
+TOML11_INLINE std::string to_string(const integer_format c)
+{
+    std::ostringstream oss;
+    oss << c;
+    return oss.str();
+}
+
+
+TOML11_INLINE bool operator==(const integer_format_info& lhs, const integer_format_info& rhs) noexcept
+{
+    return lhs.fmt       == rhs.fmt    &&
+           lhs.uppercase == rhs.uppercase &&
+           lhs.width     == rhs.width  &&
+           lhs.spacer    == rhs.spacer &&
+           lhs.suffix    == rhs.suffix ;
+}
+TOML11_INLINE bool operator!=(const integer_format_info& lhs, const integer_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+// ----------------------------------------------------------------------------
+// floating
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const floating_format f)
+{
+    switch(f)
+    {
+        case floating_format::defaultfloat: {os << "defaultfloat"; break;}
+        case floating_format::fixed       : {os << "fixed"       ; break;}
+        case floating_format::scientific  : {os << "scientific"  ; break;}
+        case floating_format::hex         : {os << "hex"         ; break;}
+        default:
+        {
+            os << "unknown floating_format: " << static_cast<std::uint8_t>(f);
+            break;
+        }
+    }
+    return os;
+}
+TOML11_INLINE std::string to_string(const floating_format c)
+{
+    std::ostringstream oss;
+    oss << c;
+    return oss.str();
+}
+
+TOML11_INLINE bool operator==(const floating_format_info& lhs, const floating_format_info& rhs) noexcept
+{
+    return lhs.fmt          == rhs.fmt          &&
+           lhs.prec         == rhs.prec         &&
+           lhs.suffix       == rhs.suffix       ;
+}
+TOML11_INLINE bool operator!=(const floating_format_info& lhs, const floating_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+// ----------------------------------------------------------------------------
+// string
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const string_format f)
+{
+    switch(f)
+    {
+        case string_format::basic            : {os << "basic"            ; break;}
+        case string_format::literal          : {os << "literal"          ; break;}
+        case string_format::multiline_basic  : {os << "multiline_basic"  ; break;}
+        case string_format::multiline_literal: {os << "multiline_literal"; break;}
+        default:
+        {
+            os << "unknown string_format: " << static_cast<std::uint8_t>(f);
+            break;
+        }
+    }
+    return os;
+}
+TOML11_INLINE std::string to_string(const string_format c)
+{
+    std::ostringstream oss;
+    oss << c;
+    return oss.str();
+}
+
+TOML11_INLINE bool operator==(const string_format_info& lhs, const string_format_info& rhs) noexcept
+{
+    return lhs.fmt                == rhs.fmt                &&
+           lhs.start_with_newline == rhs.start_with_newline ;
+}
+TOML11_INLINE bool operator!=(const string_format_info& lhs, const string_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+// ----------------------------------------------------------------------------
+// datetime
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const datetime_delimiter_kind d)
+{
+    switch(d)
+    {
+        case datetime_delimiter_kind::upper_T: { os << "upper_T, "; break; }
+        case datetime_delimiter_kind::lower_t: { os << "lower_t, "; break; }
+        case datetime_delimiter_kind::space:   { os << "space, ";   break; }
+        default:
+        {
+            os << "unknown datetime delimiter: " << static_cast<std::uint8_t>(d);
+            break;
+        }
+    }
+    return os;
+}
+TOML11_INLINE std::string to_string(const datetime_delimiter_kind c)
+{
+    std::ostringstream oss;
+    oss << c;
+    return oss.str();
+}
+
+TOML11_INLINE bool operator==(const offset_datetime_format_info& lhs, const offset_datetime_format_info& rhs) noexcept
+{
+    return lhs.delimiter           == rhs.delimiter   &&
+           lhs.has_seconds         == rhs.has_seconds &&
+           lhs.subsecond_precision == rhs.subsecond_precision ;
+}
+TOML11_INLINE bool operator!=(const offset_datetime_format_info& lhs, const offset_datetime_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+TOML11_INLINE bool operator==(const local_datetime_format_info& lhs, const local_datetime_format_info& rhs) noexcept
+{
+    return lhs.delimiter           == rhs.delimiter   &&
+           lhs.has_seconds         == rhs.has_seconds &&
+           lhs.subsecond_precision == rhs.subsecond_precision ;
+}
+TOML11_INLINE bool operator!=(const local_datetime_format_info& lhs, const local_datetime_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+TOML11_INLINE bool operator==(const local_date_format_info&, const local_date_format_info&) noexcept
+{
+    return true;
+}
+TOML11_INLINE bool operator!=(const local_date_format_info& lhs, const local_date_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+TOML11_INLINE bool operator==(const local_time_format_info& lhs, const local_time_format_info& rhs) noexcept
+{
+    return lhs.has_seconds         == rhs.has_seconds         &&
+           lhs.subsecond_precision == rhs.subsecond_precision ;
+}
+TOML11_INLINE bool operator!=(const local_time_format_info& lhs, const local_time_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+// ----------------------------------------------------------------------------
+// array
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const array_format f)
+{
+    switch(f)
+    {
+        case array_format::default_format : {os << "default_format" ; break;}
+        case array_format::oneline        : {os << "oneline"        ; break;}
+        case array_format::multiline      : {os << "multiline"      ; break;}
+        case array_format::array_of_tables: {os << "array_of_tables"; break;}
+        default:
+        {
+            os << "unknown array_format: " << static_cast<std::uint8_t>(f);
+            break;
+        }
+    }
+    return os;
+}
+TOML11_INLINE std::string to_string(const array_format c)
+{
+    std::ostringstream oss;
+    oss << c;
+    return oss.str();
+}
+
+TOML11_INLINE bool operator==(const array_format_info& lhs, const array_format_info& rhs) noexcept
+{
+    return lhs.fmt            == rhs.fmt            &&
+           lhs.indent_type    == rhs.indent_type    &&
+           lhs.body_indent    == rhs.body_indent    &&
+           lhs.closing_indent == rhs.closing_indent ;
+}
+TOML11_INLINE bool operator!=(const array_format_info& lhs, const array_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+// ----------------------------------------------------------------------------
+// table
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const table_format f)
+{
+    switch(f)
+    {
+        case table_format::multiline        : {os << "multiline"        ; break;}
+        case table_format::oneline          : {os << "oneline"          ; break;}
+        case table_format::dotted           : {os << "dotted"           ; break;}
+        case table_format::multiline_oneline: {os << "multiline_oneline"; break;}
+        case table_format::implicit         : {os << "implicit"         ; break;}
+        default:
+        {
+            os << "unknown table_format: " << static_cast<std::uint8_t>(f);
+            break;
+        }
+    }
+    return os;
+}
+TOML11_INLINE std::string to_string(const table_format c)
+{
+    std::ostringstream oss;
+    oss << c;
+    return oss.str();
+}
+
+TOML11_INLINE bool operator==(const table_format_info& lhs, const table_format_info& rhs) noexcept
+{
+    return lhs.fmt                == rhs.fmt              &&
+           lhs.indent_type        == rhs.indent_type      &&
+           lhs.body_indent        == rhs.body_indent      &&
+           lhs.name_indent        == rhs.name_indent      &&
+           lhs.closing_indent     == rhs.closing_indent   ;
+}
+TOML11_INLINE bool operator!=(const table_format_info& lhs, const table_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
+} // namespace toml
+#endif // TOML11_FORMAT_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/literal_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/literal_impl.hpp
new file mode 100644
index 00000000..e8298c21
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/literal_impl.hpp
@@ -0,0 +1,174 @@
+#ifndef TOML11_LITERAL_IMPL_HPP
+#define TOML11_LITERAL_IMPL_HPP
+
+#include "../fwd/literal_fwd.hpp"
+#include "../parser.hpp"
+#include "../syntax.hpp"
+
+namespace toml
+{
+
+namespace detail
+{
+// implementation
+TOML11_INLINE ::toml::value literal_internal_impl(location loc)
+{
+    const auto s = ::toml::spec::default_version();
+    context<type_config> ctx(s);
+
+    const auto front = loc;
+
+    // ------------------------------------------------------------------------
+    // check if it is a raw value.
+
+    // skip empty lines and comment lines
+    auto sp = skip_multiline_spacer(loc, ctx);
+    if(loc.eof())
+    {
+        ::toml::value val;
+        if(sp.has_value())
+        {
+            for(std::size_t i=0; i<sp.value().comments.size(); ++i)
+            {
+                val.comments().push_back(std::move(sp.value().comments.at(i)));
+            }
+        }
+        return val;
+    }
+
+    // to distinguish arrays and tables, first check it is a table or not.
+    //
+    // "[1,2,3]"_toml;   // json: [1, 2, 3]
+    // "[table]"_toml;   // json: {"table": {}}
+    // "[[1,2,3]]"_toml; // json: [[1, 2, 3]]
+    // "[[table]]"_toml; // json: {"table": [{}]}
+    //
+    // "[[1]]"_toml;     // json: {"1": [{}]}
+    // "1 = [{}]"_toml;  // json: {"1": [{}]}
+    // "[[1,]]"_toml;    // json: [[1]]
+    // "[[1],]"_toml;    // json: [[1]]
+    const auto val_start = loc;
+
+    const bool is_table_key = syntax::std_table(s).scan(loc).is_ok();
+    loc = val_start;
+    const bool is_aots_key  = syntax::array_table(s).scan(loc).is_ok();
+    loc = val_start;
+
+    // If it is neither a table-key or a array-of-table-key, it may be a value.
+    if(!is_table_key && !is_aots_key)
+    {
+        auto data = parse_value(loc, ctx);
+        if(data.is_ok())
+        {
+            auto val = std::move(data.unwrap());
+            if(sp.has_value())
+            {
+                for(std::size_t i=0; i<sp.value().comments.size(); ++i)
+                {
+                    val.comments().push_back(std::move(sp.value().comments.at(i)));
+                }
+            }
+            auto com_res = parse_comment_line(loc, ctx);
+            if(com_res.is_ok() && com_res.unwrap().has_value())
+            {
+                val.comments().push_back(com_res.unwrap().value());
+            }
+            return val;
+        }
+    }
+
+    // -------------------------------------------------------------------------
+    // Note that still it can be a table, because the literal might be something
+    // like the following.
+    // ```cpp
+    // // c++11 raw-string literal
+    // const auto val = R"(
+    //   key = "value"
+    //   int = 42
+    // )"_toml;
+    // ```
+    // It is a valid toml file.
+    // It should be parsed as if we parse a file with this content.
+
+    loc = front;
+    auto data = parse_file(loc, ctx);
+    if(data.is_ok())
+    {
+        return data.unwrap();
+    }
+    else // not a value && not a file. error.
+    {
+        std::string msg;
+        for(const auto& err : data.unwrap_err())
+        {
+            msg += format_error(err);
+        }
+        throw ::toml::syntax_error(std::move(msg), std::move(data.unwrap_err()));
+    }
+}
+
+} // detail
+
+inline namespace literals
+{
+inline namespace toml_literals
+{
+
+TOML11_INLINE ::toml::value
+operator"" _toml(const char* str, std::size_t len)
+{
+    if(len == 0)
+    {
+        return ::toml::value{};
+    }
+
+    ::toml::detail::location::container_type c(len);
+    std::copy(reinterpret_cast<const ::toml::detail::location::char_type*>(str),
+              reinterpret_cast<const ::toml::detail::location::char_type*>(str + len),
+              c.begin());
+    if( ! c.empty() && c.back())
+    {
+        c.push_back('\n'); // to make it easy to parse comment, we add newline
+    }
+
+    return literal_internal_impl(::toml::detail::location(
+        std::make_shared<const toml::detail::location::container_type>(std::move(c)),
+        "TOML literal encoded in a C++ code"));
+}
+
+#if defined(__cpp_char8_t)
+#  if __cpp_char8_t >= 201811L
+#    define TOML11_HAS_CHAR8_T 1
+#  endif
+#endif
+
+#if defined(TOML11_HAS_CHAR8_T)
+// value of u8"" literal has been changed from char to char8_t and char8_t is
+// NOT compatible to char
+TOML11_INLINE ::toml::value
+operator"" _toml(const char8_t* str, std::size_t len)
+{
+    if(len == 0)
+    {
+        return ::toml::value{};
+    }
+
+    ::toml::detail::location::container_type c(len);
+    std::copy(reinterpret_cast<const ::toml::detail::location::char_type*>(str),
+              reinterpret_cast<const ::toml::detail::location::char_type*>(str + len),
+              c.begin());
+    if( ! c.empty() && c.back())
+    {
+        c.push_back('\n'); // to make it easy to parse comment, we add newline
+    }
+
+    return literal_internal_impl(::toml::detail::location(
+        std::make_shared<const toml::detail::location::container_type>(std::move(c)),
+        "TOML literal encoded in a C++ code"));
+}
+#endif
+
+} // toml_literals
+} // literals
+} // toml
+#endif // TOML11_LITERAL_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/location_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/location_impl.hpp
new file mode 100644
index 00000000..19c418ab
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/location_impl.hpp
@@ -0,0 +1,226 @@
+#ifndef TOML11_LOCATION_IMPL_HPP
+#define TOML11_LOCATION_IMPL_HPP
+
+#include "../fwd/location_fwd.hpp"
+#include "../utility.hpp"
+#include "../version.hpp"
+
+namespace toml
+{
+namespace detail
+{
+
+TOML11_INLINE void location::advance(std::size_t n) noexcept
+{
+    assert(this->is_ok());
+    if(this->location_ + n < this->source_->size())
+    {
+        this->advance_line_number(n);
+        this->location_ += n;
+    }
+    else
+    {
+        this->advance_line_number(this->source_->size() - this->location_);
+        this->location_ = this->source_->size();
+    }
+}
+TOML11_INLINE void location::retrace(std::size_t n) noexcept
+{
+    assert(this->is_ok());
+    if(this->location_ < n)
+    {
+        this->location_ = 0;
+        this->line_number_ = 1;
+    }
+    else
+    {
+        this->retrace_line_number(n);
+        this->location_ -= n;
+    }
+}
+
+TOML11_INLINE bool location::eof() const noexcept
+{
+    assert(this->is_ok());
+    return this->location_ >= this->source_->size();
+}
+TOML11_INLINE location::char_type location::current() const
+{
+    assert(this->is_ok());
+    if(this->eof()) {return '\0';}
+
+    assert(this->location_ < this->source_->size());
+    return this->source_->at(this->location_);
+}
+
+TOML11_INLINE location::char_type location::peek()
+{
+    assert(this->is_ok());
+    if(this->location_ >= this->source_->size())
+    {
+        return '\0';
+    }
+    else
+    {
+        return this->source_->at(this->location_ + 1);
+    }
+}
+
+TOML11_INLINE void location::set_location(const std::size_t loc) noexcept
+{
+    if(this->location_ == loc)
+    {
+        return ;
+    }
+
+    if(loc == 0)
+    {
+        this->line_number_ = 1;
+    }
+    else if(this->location_ < loc)
+    {
+        const auto d = loc - this->location_;
+        this->advance_line_number(d);
+    }
+    else
+    {
+        const auto d = this->location_ - loc;
+        this->retrace_line_number(d);
+    }
+    this->location_ = loc;
+}
+
+TOML11_INLINE std::string location::get_line() const
+{
+    assert(this->is_ok());
+    const auto iter = std::next(this->source_->cbegin(), static_cast<difference_type>(this->location_));
+    const auto riter = cxx::make_reverse_iterator(iter);
+
+    const auto prev = std::find(riter, this->source_->crend(), char_type('\n'));
+    const auto next = std::find(iter,  this->source_->cend(),  char_type('\n'));
+
+    return make_string(std::next(prev.base()), next);
+}
+TOML11_INLINE std::size_t location::column_number() const noexcept
+{
+    assert(this->is_ok());
+    const auto iter  = std::next(this->source_->cbegin(), static_cast<difference_type>(this->location_));
+    const auto riter = cxx::make_reverse_iterator(iter);
+    const auto prev  = std::find(riter, this->source_->crend(), char_type('\n'));
+
+    assert(prev.base() <= iter);
+    return static_cast<std::size_t>(std::distance(prev.base(), iter) + 1); // 1-origin
+}
+
+
+TOML11_INLINE void location::advance_line_number(const std::size_t n)
+{
+    assert(this->is_ok());
+    assert(this->location_ + n <= this->source_->size());
+
+    const auto iter = this->source_->cbegin();
+    this->line_number_ += static_cast<std::size_t>(std::count(
+        std::next(iter, static_cast<difference_type>(this->location_)),
+        std::next(iter, static_cast<difference_type>(this->location_ + n)),
+        char_type('\n')));
+
+    return;
+}
+TOML11_INLINE void location::retrace_line_number(const std::size_t n)
+{
+    assert(this->is_ok());
+    assert(n <= this->location_); // loc - n >= 0
+
+    const auto iter = this->source_->cbegin();
+    const auto dline_num = static_cast<std::size_t>(std::count(
+        std::next(iter, static_cast<difference_type>(this->location_ - n)),
+        std::next(iter, static_cast<difference_type>(this->location_)),
+        char_type('\n')));
+
+    if(this->line_number_ <= dline_num)
+    {
+        this->line_number_ = 1;
+    }
+    else
+    {
+        this->line_number_ -= dline_num;
+    }
+    return;
+}
+
+TOML11_INLINE bool operator==(const location& lhs, const location& rhs) noexcept
+{
+    if( ! lhs.is_ok() || ! rhs.is_ok())
+    {
+        return (!lhs.is_ok()) && (!rhs.is_ok());
+    }
+    return lhs.source()       == rhs.source()      &&
+           lhs.source_name()  == rhs.source_name() &&
+           lhs.get_location() == rhs.get_location();
+}
+TOML11_INLINE bool operator!=(const location& lhs, const location& rhs)
+{
+    return !(lhs == rhs);
+}
+
+TOML11_INLINE location prev(const location& loc)
+{
+    location p(loc);
+    p.retrace(1);
+    return p;
+}
+TOML11_INLINE location next(const location& loc)
+{
+    location p(loc);
+    p.advance(1);
+    return p;
+}
+
+TOML11_INLINE location make_temporary_location(const std::string& str) noexcept
+{
+    location::container_type cont(str.size());
+    std::transform(str.begin(), str.end(), cont.begin(),
+        [](const std::string::value_type& c) {
+            return cxx::bit_cast<location::char_type>(c);
+        });
+    return location(std::make_shared<const location::container_type>(
+            std::move(cont)), "internal temporary");
+}
+
+TOML11_INLINE result<location, none_t>
+find(const location& first, const location& last, const location::char_type val)
+{
+    return find_if(first, last, [val](const location::char_type c) {
+            return c == val;
+        });
+}
+TOML11_INLINE result<location, none_t>
+rfind(const location& first, const location& last, const location::char_type val)
+{
+    return rfind_if(first, last, [val](const location::char_type c) {
+            return c == val;
+        });
+}
+
+TOML11_INLINE std::size_t
+count(const location& first, const location& last, const location::char_type& c)
+{
+    if(first.source() != last.source())             { return 0; }
+    if(first.get_location() >= last.get_location()) { return 0; }
+
+    auto loc = first;
+    std::size_t num = 0;
+    while(loc.get_location() != last.get_location())
+    {
+        if(loc.current() == c)
+        {
+            num += 1;
+        }
+        loc.advance();
+    }
+    return num;
+}
+
+} // detail
+} // toml
+#endif // TOML11_LOCATION_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/region_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/region_impl.hpp
new file mode 100644
index 00000000..08147e25
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/region_impl.hpp
@@ -0,0 +1,188 @@
+#ifndef TOML11_REGION_IMPL_HPP
+#define TOML11_REGION_IMPL_HPP
+
+#include "../fwd/region_fwd.hpp"
+#include "../utility.hpp"
+
+#include <algorithm>
+#include <iterator>
+#include <string>
+#include <sstream>
+#include <vector>
+#include <cassert>
+
+namespace toml
+{
+namespace detail
+{
+
+// a value defined in [first, last).
+// Those source must be the same. Instread, `region` does not make sense.
+TOML11_INLINE region::region(const location& first, const location& last)
+    : source_(first.source()), source_name_(first.source_name()),
+      length_(last.get_location() - first.get_location()),
+      first_(first.get_location()),
+      first_line_(first.line_number()),
+      first_column_(first.column_number()),
+      last_(last.get_location()),
+      last_line_(last.line_number()),
+      last_column_(last.column_number())
+{
+    assert(first.source()      == last.source());
+    assert(first.source_name() == last.source_name());
+}
+
+    // shorthand of [loc, loc+1)
+TOML11_INLINE region::region(const location& loc)
+    : source_(loc.source()), source_name_(loc.source_name()), length_(0),
+      first_line_(0), first_column_(0), last_line_(0), last_column_(0)
+{
+    // if the file ends with LF, the resulting region points no char.
+    if(loc.eof())
+    {
+        if(loc.get_location() == 0)
+        {
+            this->length_       = 0;
+            this->first_        = 0;
+            this->first_line_   = 0;
+            this->first_column_ = 0;
+            this->last_         = 0;
+            this->last_line_    = 0;
+            this->last_column_  = 0;
+        }
+        else
+        {
+            const auto first = prev(loc);
+            this->first_        = first.get_location();
+            this->first_line_   = first.line_number();
+            this->first_column_ = first.column_number();
+            this->last_         = loc.get_location();
+            this->last_line_    = loc.line_number();
+            this->last_column_  = loc.column_number();
+            this->length_       = 1;
+        }
+    }
+    else
+    {
+        this->first_        = loc.get_location();
+        this->first_line_   = loc.line_number();
+        this->first_column_ = loc.column_number();
+        this->last_         = loc.get_location() + 1;
+        this->last_line_    = loc.line_number();
+        this->last_column_  = loc.column_number() + 1;
+        this->length_       = 1;
+    }
+}
+
+TOML11_INLINE region::char_type region::at(std::size_t i) const
+{
+    if(this->last_ <= this->first_ + i)
+    {
+        throw std::out_of_range("range::at: index " + std::to_string(i) +
+                " exceeds length " + std::to_string(this->length_));
+    }
+    const auto iter = std::next(this->source_->cbegin(),
+            static_cast<difference_type>(this->first_ + i));
+    return *iter;
+}
+
+TOML11_INLINE region::const_iterator region::begin() const noexcept
+{
+    return std::next(this->source_->cbegin(),
+            static_cast<difference_type>(this->first_));
+}
+TOML11_INLINE region::const_iterator region::end() const noexcept
+{
+    return std::next(this->source_->cbegin(),
+            static_cast<difference_type>(this->last_));
+}
+TOML11_INLINE region::const_iterator region::cbegin() const noexcept
+{
+    return std::next(this->source_->cbegin(),
+            static_cast<difference_type>(this->first_));
+}
+TOML11_INLINE region::const_iterator region::cend() const noexcept
+{
+    return std::next(this->source_->cbegin(),
+            static_cast<difference_type>(this->last_));
+}
+
+TOML11_INLINE std::string region::as_string() const
+{
+    if(this->is_ok())
+    {
+        const auto begin = std::next(this->source_->cbegin(), static_cast<difference_type>(this->first_));
+        const auto end   = std::next(this->source_->cbegin(), static_cast<difference_type>(this->last_ ));
+        return ::toml::detail::make_string(begin, end);
+    }
+    else
+    {
+        return std::string("");
+    }
+}
+
+TOML11_INLINE std::vector<std::string> region::as_lines() const
+{
+    assert(this->is_ok());
+    if(this->length_ == 0)
+    {
+        return std::vector<std::string>{""};
+    }
+
+    // Consider the following toml file
+    // ```
+    // array = [
+    // ] # comment
+    // ```
+    // and the region represnets
+    // ```
+    //         [
+    // ]
+    // ```
+    // but we want to show the following.
+    // ```
+    // array = [
+    // ] # comment
+    // ```
+    // So we need to find LFs before `begin` and after `end`.
+    //
+    // But, if region ends with LF, it should not include the next line.
+    // ```
+    // a = 42
+    //     ^^^- with the last LF
+    // ```
+    // So we start from `end-1` when looking for LF.
+
+    const auto begin_idx = static_cast<difference_type>(this->first_);
+    const auto end_idx   = static_cast<difference_type>(this->last_) - 1;
+
+    // length_ != 0, so begin < end. then begin <= end-1
+    assert(begin_idx <= end_idx);
+
+    const auto begin = std::next(this->source_->cbegin(), begin_idx);
+    const auto end   = std::next(this->source_->cbegin(), end_idx);
+
+    const auto line_begin = std::find(cxx::make_reverse_iterator(begin), this->source_->crend(), char_type('\n')).base();
+    const auto line_end   = std::find(end, this->source_->cend(), char_type('\n'));
+
+    const auto reg_lines = make_string(line_begin, line_end);
+
+    if(reg_lines == "") // the region is an empty line that only contains LF
+    {
+        return std::vector<std::string>{""};
+    }
+
+    std::istringstream iss(reg_lines);
+
+    std::vector<std::string> lines;
+    std::string line;
+    while(std::getline(iss, line))
+    {
+        lines.push_back(line);
+    }
+    return lines;
+}
+
+} // namespace detail
+} // namespace toml
+#endif // TOML11_REGION_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/scanner_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/scanner_impl.hpp
new file mode 100644
index 00000000..d97d3178
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/scanner_impl.hpp
@@ -0,0 +1,473 @@
+#ifndef TOML11_SCANNER_IMPL_HPP
+#define TOML11_SCANNER_IMPL_HPP
+
+#include "../fwd/scanner_fwd.hpp"
+#include "../utility.hpp"
+
+namespace toml
+{
+namespace detail
+{
+
+TOML11_INLINE scanner_storage::scanner_storage(const scanner_storage& other)
+    : scanner_(nullptr)
+{
+    if(other.is_ok())
+    {
+        scanner_.reset(other.get().clone());
+    }
+}
+TOML11_INLINE scanner_storage& scanner_storage::operator=(const scanner_storage& other)
+{
+    if(this == std::addressof(other)) {return *this;}
+    if(other.is_ok())
+    {
+        scanner_.reset(other.get().clone());
+    }
+    return *this;
+}
+
+TOML11_INLINE region scanner_storage::scan(location& loc) const
+{
+    assert(this->is_ok());
+    return this->scanner_->scan(loc);
+}
+
+TOML11_INLINE std::string scanner_storage::expected_chars(location& loc) const
+{
+    assert(this->is_ok());
+    return this->scanner_->expected_chars(loc);
+}
+
+TOML11_INLINE scanner_base& scanner_storage::get() const noexcept
+{
+    assert(this->is_ok());
+    return *scanner_;
+}
+
+TOML11_INLINE std::string scanner_storage::name() const
+{
+    assert(this->is_ok());
+    return this->scanner_->name();
+}
+
+// ----------------------------------------------------------------------------
+
+TOML11_INLINE region character::scan(location& loc) const
+{
+    if(loc.eof()) {return region{};}
+
+    if(loc.current() == this->value_)
+    {
+        const auto first = loc;
+        loc.advance(1);
+        return region(first, loc);
+    }
+    return region{};
+}
+
+TOML11_INLINE std::string character::expected_chars(location&) const
+{
+    return show_char(value_);
+}
+
+TOML11_INLINE scanner_base* character::clone() const
+{
+    return new character(*this);
+}
+
+TOML11_INLINE std::string character::name() const
+{
+    return "character{" + show_char(value_) + "}";
+}
+
+// ----------------------------------------------------------------------------
+
+TOML11_INLINE region character_either::scan(location& loc) const
+{
+    if(loc.eof()) {return region{};}
+
+    for(const auto c : this->chars_)
+    {
+        if(loc.current() == c)
+        {
+            const auto first = loc;
+            loc.advance(1);
+            return region(first, loc);
+        }
+    }
+    return region{};
+}
+
+TOML11_INLINE std::string character_either::expected_chars(location&) const
+{
+    assert( ! chars_.empty());
+
+    std::string expected;
+    if(chars_.size() == 1)
+    {
+        expected += show_char(chars_.at(0));
+    }
+    else if(chars_.size() == 2)
+    {
+        expected += show_char(chars_.at(0)) + " or " + show_char(chars_.at(1));
+    }
+    else
+    {
+        for(std::size_t i=0; i<chars_.size(); ++i)
+        {
+            if(i != 0)
+            {
+                expected += ", ";
+            }
+            if(i + 1 == chars_.size())
+            {
+                expected += "or ";
+            }
+            expected += show_char(chars_.at(i));
+        }
+    }
+    return expected;
+}
+
+TOML11_INLINE scanner_base* character_either::clone() const
+{
+    return new character_either(*this);
+}
+
+TOML11_INLINE void character_either::push_back(const char_type c)
+{
+    chars_.push_back(c);
+}
+
+TOML11_INLINE std::string character_either::name() const
+{
+    std::string n("character_either{");
+    for(const auto c : this->chars_)
+    {
+        n += show_char(c);
+        n += ", ";
+    }
+    if( ! this->chars_.empty())
+    {
+        n.pop_back();
+        n.pop_back();
+    }
+    n += "}";
+    return n;
+}
+
+// ----------------------------------------------------------------------------
+// character_in_range
+
+TOML11_INLINE region character_in_range::scan(location& loc) const
+{
+    if(loc.eof()) {return region{};}
+
+    const auto curr = loc.current();
+    if(this->from_ <= curr && curr <= this->to_)
+    {
+        const auto first = loc;
+        loc.advance(1);
+        return region(first, loc);
+    }
+    return region{};
+}
+
+TOML11_INLINE std::string character_in_range::expected_chars(location&) const
+{
+    std::string expected("from `");
+    expected += show_char(from_);
+    expected += "` to `";
+    expected += show_char(to_);
+    expected += "`";
+    return expected;
+}
+
+TOML11_INLINE scanner_base* character_in_range::clone() const
+{
+    return new character_in_range(*this);
+}
+
+TOML11_INLINE std::string character_in_range::name() const
+{
+    return "character_in_range{" + show_char(from_) + "," + show_char(to_) + "}";
+}
+
+// ----------------------------------------------------------------------------
+// literal
+
+TOML11_INLINE region literal::scan(location& loc) const
+{
+    const auto first = loc;
+    for(std::size_t i=0; i<size_; ++i)
+    {
+        if(loc.eof() || char_type(value_[i]) != loc.current())
+        {
+            loc = first;
+            return region{};
+        }
+        loc.advance(1);
+    }
+    return region(first, loc);
+}
+
+TOML11_INLINE std::string literal::expected_chars(location&) const
+{
+    return std::string(value_);
+}
+
+TOML11_INLINE scanner_base* literal::clone() const
+{
+    return new literal(*this);
+}
+
+TOML11_INLINE std::string literal::name() const
+{
+    return std::string("literal{") + std::string(value_, size_) + "}";
+}
+
+// ----------------------------------------------------------------------------
+// sequence
+
+TOML11_INLINE region sequence::scan(location& loc) const
+{
+    const auto first = loc;
+    for(const auto& other : others_)
+    {
+        const auto reg = other.scan(loc);
+        if( ! reg.is_ok())
+        {
+            loc = first;
+            return region{};
+        }
+    }
+    return region(first, loc);
+}
+
+TOML11_INLINE std::string sequence::expected_chars(location& loc) const
+{
+    const auto first = loc;
+    for(const auto& other : others_)
+    {
+        const auto reg = other.scan(loc);
+        if( ! reg.is_ok())
+        {
+            return other.expected_chars(loc);
+        }
+    }
+    assert(false);
+    return ""; // XXX
+}
+
+TOML11_INLINE scanner_base* sequence::clone() const
+{
+    return new sequence(*this);
+}
+
+TOML11_INLINE std::string sequence::name() const
+{
+    std::string n("sequence{");
+    for(const auto& other : others_)
+    {
+        n += other.name();
+        n += ", ";
+    }
+    if( ! this->others_.empty())
+    {
+        n.pop_back();
+        n.pop_back();
+    }
+    n += "}";
+    return n;
+}
+
+// ----------------------------------------------------------------------------
+// either
+
+TOML11_INLINE region either::scan(location& loc) const
+{
+    for(const auto& other : others_)
+    {
+        const auto reg = other.scan(loc);
+        if(reg.is_ok())
+        {
+            return reg;
+        }
+    }
+    return region{};
+}
+
+TOML11_INLINE std::string either::expected_chars(location& loc) const
+{
+    assert( ! others_.empty());
+
+    std::string expected = others_.at(0).expected_chars(loc);
+    if(others_.size() == 2)
+    {
+        expected += " or ";
+        expected += others_.at(1).expected_chars(loc);
+    }
+    else
+    {
+        for(std::size_t i=1; i<others_.size(); ++i)
+        {
+            expected += ", ";
+            if(i + 1 == others_.size())
+            {
+                expected += "or ";
+            }
+            expected += others_.at(i).expected_chars(loc);
+        }
+    }
+    return expected;
+}
+
+TOML11_INLINE scanner_base* either::clone() const
+{
+    return new either(*this);
+}
+
+TOML11_INLINE std::string either::name() const
+{
+    std::string n("either{");
+    for(const auto& other : others_)
+    {
+        n += other.name();
+        n += ", ";
+    }
+    if( ! this->others_.empty())
+    {
+        n.pop_back();
+        n.pop_back();
+    }
+    n += "}";
+    return n;
+}
+
+// ----------------------------------------------------------------------------
+// repeat_exact
+
+TOML11_INLINE region repeat_exact::scan(location& loc) const
+{
+    const auto first = loc;
+    for(std::size_t i=0; i<length_; ++i)
+    {
+        const auto reg = other_.scan(loc);
+        if( ! reg.is_ok())
+        {
+            loc = first;
+            return region{};
+        }
+    }
+    return region(first, loc);
+}
+
+TOML11_INLINE std::string repeat_exact::expected_chars(location& loc) const
+{
+    for(std::size_t i=0; i<length_; ++i)
+    {
+        const auto reg = other_.scan(loc);
+        if( ! reg.is_ok())
+        {
+            return other_.expected_chars(loc);
+        }
+    }
+    assert(false);
+    return "";
+}
+
+TOML11_INLINE scanner_base* repeat_exact::clone() const
+{
+    return new repeat_exact(*this);
+}
+
+TOML11_INLINE std::string repeat_exact::name() const
+{
+    return "repeat_exact{" + std::to_string(length_) + ", " + other_.name() + "}";
+}
+
+// ----------------------------------------------------------------------------
+// repeat_at_least
+
+TOML11_INLINE region repeat_at_least::scan(location& loc) const
+{
+    const auto first = loc;
+    for(std::size_t i=0; i<length_; ++i)
+    {
+        const auto reg = other_.scan(loc);
+        if( ! reg.is_ok())
+        {
+            loc = first;
+            return region{};
+        }
+    }
+    while( ! loc.eof())
+    {
+        const auto checkpoint = loc;
+        const auto reg = other_.scan(loc);
+        if( ! reg.is_ok())
+        {
+            loc = checkpoint;
+            return region(first, loc);
+        }
+    }
+    return region(first, loc);
+}
+
+TOML11_INLINE std::string repeat_at_least::expected_chars(location& loc) const
+{
+    for(std::size_t i=0; i<length_; ++i)
+    {
+        const auto reg = other_.scan(loc);
+        if( ! reg.is_ok())
+        {
+            return other_.expected_chars(loc);
+        }
+    }
+    assert(false);
+    return "";
+}
+
+TOML11_INLINE scanner_base* repeat_at_least::clone() const
+{
+    return new repeat_at_least(*this);
+}
+
+TOML11_INLINE std::string repeat_at_least::name() const
+{
+    return "repeat_at_least{" + std::to_string(length_) + ", " + other_.name() + "}";
+}
+
+// ----------------------------------------------------------------------------
+// maybe
+
+TOML11_INLINE region maybe::scan(location& loc) const
+{
+    const auto first = loc;
+    const auto reg = other_.scan(loc);
+    if( ! reg.is_ok())
+    {
+        loc = first;
+    }
+    return region(first, loc);
+}
+
+TOML11_INLINE std::string maybe::expected_chars(location&) const
+{
+    return "";
+}
+
+TOML11_INLINE scanner_base* maybe::clone() const
+{
+    return new maybe(*this);
+}
+
+TOML11_INLINE std::string maybe::name() const
+{
+    return "maybe{" + other_.name() + "}";
+}
+
+} // detail
+} // toml
+#endif // TOML11_SCANNER_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/source_location_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/source_location_impl.hpp
new file mode 100644
index 00000000..f7f4638f
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/source_location_impl.hpp
@@ -0,0 +1,197 @@
+#ifndef TOML11_SOURCE_LOCATION_IMPL_HPP
+#define TOML11_SOURCE_LOCATION_IMPL_HPP
+
+#include "../fwd/source_location_fwd.hpp"
+
+#include "../color.hpp"
+#include "../utility.hpp"
+
+#include <iomanip>
+#include <sstream>
+#include <string>
+#include <vector>
+
+#include <cctype>
+
+namespace toml
+{
+
+TOML11_INLINE source_location::source_location(const detail::region& r)
+    : is_ok_(false),
+      first_line_(1),
+      first_column_(1),
+      last_line_(1),
+      last_column_(1),
+      length_(0),
+      file_name_("unknown file")
+{
+    if(r.is_ok())
+    {
+        this->is_ok_        = true;
+        this->file_name_    = r.source_name();
+        this->first_line_   = r.first_line_number();
+        this->first_column_ = r.first_column_number();
+        this->last_line_    = r.last_line_number();
+        this->last_column_  = r.last_column_number();
+        this->length_       = r.length();
+        this->line_str_     = r.as_lines();
+    }
+}
+
+TOML11_INLINE std::string const& source_location::first_line() const
+{
+    if(this->line_str_.size() == 0)
+    {
+        throw std::out_of_range("toml::source_location::first_line: `lines` is empty");
+    }
+    return this->line_str_.front();
+}
+TOML11_INLINE std::string const& source_location::last_line() const
+{
+    if(this->line_str_.size() == 0)
+    {
+        throw std::out_of_range("toml::source_location::first_line: `lines` is empty");
+    }
+    return this->line_str_.back();
+}
+
+namespace detail
+{
+
+TOML11_INLINE std::size_t integer_width_base10(std::size_t i) noexcept
+{
+    std::size_t width = 0;
+    while(i != 0)
+    {
+        i /= 10;
+        width += 1;
+    }
+    return width;
+}
+
+TOML11_INLINE std::ostringstream&
+format_filename(std::ostringstream& oss, const source_location& loc)
+{
+    // --> example.toml
+    oss << color::bold << color::blue << " --> " << color::reset
+        << color::bold << loc.file_name() << '\n' << color::reset;
+    return oss;
+}
+
+TOML11_INLINE std::ostringstream& format_empty_line(std::ostringstream& oss,
+        const std::size_t lnw)
+{
+    //    |
+    oss << detail::make_string(lnw + 1, ' ')
+        << color::bold << color::blue << " |\n"  << color::reset;
+    return oss;
+}
+
+TOML11_INLINE std::ostringstream& format_line(std::ostringstream& oss,
+    const std::size_t lnw, const std::size_t linenum, const std::string& line)
+{
+    // 10 | key = "value"
+    oss << ' ' << color::bold << color::blue
+        << std::setw(static_cast<int>(lnw))
+        << std::right << linenum << " | "  << color::reset;
+    for(const char c : line)
+    {
+        if(std::isgraph(c) || c == ' ')
+        {
+            oss << c;
+        }
+        else
+        {
+            oss << show_char(c);
+        }
+    }
+    oss << '\n';
+    return oss;
+}
+TOML11_INLINE std::ostringstream& format_underline(std::ostringstream& oss,
+        const std::size_t lnw, const std::size_t col, const std::size_t len,
+        const std::string& msg)
+{
+    //    |       ^^^^^^^-- this part
+    oss << make_string(lnw + 1, ' ')
+        << color::bold << color::blue << " | " << color::reset;
+
+    oss << make_string(col-1 /*1-origin*/, ' ')
+        << color::bold << color::red
+        << make_string(len, '^') << "-- "
+        << color::reset << msg << '\n';
+
+    return oss;
+}
+
+TOML11_INLINE std::string format_location_impl(const std::size_t lnw,
+    const std::string& prev_fname,
+    const source_location& loc, const std::string& msg)
+{
+    std::ostringstream oss;
+
+    if(loc.file_name() != prev_fname)
+    {
+        format_filename(oss, loc);
+        if( ! loc.lines().empty())
+        {
+            format_empty_line(oss, lnw);
+        }
+    }
+
+    if(loc.lines().size() == 1)
+    {
+        // when column points LF, it exceeds the size of the first line.
+        std::size_t underline_limit = 1;
+        if(loc.first_line().size() < loc.first_column_number())
+        {
+            underline_limit = 1;
+        }
+        else
+        {
+            underline_limit = loc.first_line().size() - loc.first_column_number() + 1;
+        }
+        const auto underline_len = (std::min)(underline_limit, loc.length());
+
+        format_line(oss, lnw, loc.first_line_number(), loc.first_line());
+        format_underline(oss, lnw, loc.first_column_number(), underline_len, msg);
+    }
+    else if(loc.lines().size() == 2)
+    {
+        const auto first_underline_len =
+            loc.first_line().size() - loc.first_column_number() + 1;
+        format_line(oss, lnw, loc.first_line_number(), loc.first_line());
+        format_underline(oss, lnw, loc.first_column_number(),
+                first_underline_len, "");
+
+        format_line(oss, lnw, loc.last_line_number(), loc.last_line());
+        format_underline(oss, lnw, 1, loc.last_column_number(), msg);
+    }
+    else if(loc.lines().size() > 2)
+    {
+        const auto first_underline_len =
+            loc.first_line().size() - loc.first_column_number() + 1;
+        format_line(oss, lnw, loc.first_line_number(), loc.first_line());
+        format_underline(oss, lnw, loc.first_column_number(),
+                first_underline_len, "and");
+
+        if(loc.lines().size() == 3)
+        {
+            format_line(oss, lnw, loc.first_line_number()+1, loc.lines().at(1));
+            format_underline(oss, lnw, 1, loc.lines().at(1).size(), "and");
+        }
+        else
+        {
+            format_line(oss, lnw, loc.first_line_number()+1, " ...");
+            format_empty_line(oss, lnw);
+        }
+        format_line(oss, lnw, loc.last_line_number(), loc.last_line());
+        format_underline(oss, lnw, 1, loc.last_column_number(), msg);
+    }
+    // if loc is empty, do nothing.
+    return oss.str();
+}
+
+} // namespace detail
+} // toml
+#endif // TOML11_SOURCE_LOCATION_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/syntax_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/syntax_impl.hpp
new file mode 100644
index 00000000..7dd3b81f
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/syntax_impl.hpp
@@ -0,0 +1,732 @@
+#ifndef TOML11_SYNTAX_IMPL_HPP
+#define TOML11_SYNTAX_IMPL_HPP
+
+#include "../fwd/syntax_fwd.hpp"
+#include "../scanner.hpp"
+#include "../spec.hpp"
+
+namespace toml
+{
+namespace detail
+{
+namespace syntax
+{
+
+using char_type = location::char_type;
+
+// ===========================================================================
+// UTF-8
+
+// avoid redundant representation and out-of-unicode sequence
+
+TOML11_INLINE character_in_range utf8_1byte(const spec&)
+{
+    return character_in_range(0x00, 0x7F);
+}
+
+TOML11_INLINE sequence utf8_2bytes(const spec&)
+{
+    return sequence(character_in_range(0xC2, 0xDF),
+                    character_in_range(0x80, 0xBF));
+}
+
+TOML11_INLINE sequence utf8_3bytes(const spec&)
+{
+    return sequence(/*1~2 bytes = */either(
+        sequence(character         (0xE0),       character_in_range(0xA0, 0xBF)),
+        sequence(character_in_range(0xE1, 0xEC), character_in_range(0x80, 0xBF)),
+        sequence(character         (0xED),       character_in_range(0x80, 0x9F)),
+        sequence(character_in_range(0xEE, 0xEF), character_in_range(0x80, 0xBF))
+    ), /*3rd byte = */ character_in_range(0x80, 0xBF));
+}
+
+TOML11_INLINE sequence utf8_4bytes(const spec&)
+{
+    return sequence(/*1~2 bytes = */either(
+        sequence(character         (0xF0),       character_in_range(0x90, 0xBF)),
+        sequence(character_in_range(0xF1, 0xF3), character_in_range(0x80, 0xBF)),
+        sequence(character         (0xF4),       character_in_range(0x80, 0x8F))
+    ), character_in_range(0x80, 0xBF), character_in_range(0x80, 0xBF));
+}
+
+TOML11_INLINE non_ascii::non_ascii(const spec& s) noexcept
+    : scanner_(utf8_2bytes(s), utf8_3bytes(s), utf8_4bytes(s))
+{}
+
+
+// ===========================================================================
+// Whitespace
+
+TOML11_INLINE character_either wschar(const spec&)
+{
+    return character_either{char_type(' '), char_type('\t')};
+}
+
+TOML11_INLINE repeat_at_least ws(const spec& s)
+{
+    return repeat_at_least(0, wschar(s));
+}
+
+// ===========================================================================
+// Newline
+
+TOML11_INLINE either newline(const spec&)
+{
+    return either(character(char_type('\n')), literal("\r\n"));
+}
+
+// ===========================================================================
+// Comments
+
+TOML11_INLINE either allowed_comment_char(const spec& s)
+{
+    if(s.v1_1_0_allow_control_characters_in_comments)
+    {
+        return either(
+            character_in_range(0x01, 0x09),
+            character_in_range(0x0E, 0x7F),
+            non_ascii(s)
+        );
+    }
+    else
+    {
+        return either(
+            character(0x09),
+            character_in_range(0x20, 0x7E),
+            non_ascii(s)
+        );
+    }
+}
+
+// XXX Note that it does not take newline
+TOML11_INLINE sequence comment(const spec& s)
+{
+    return sequence(character(char_type('#')),
+                    repeat_at_least(0, allowed_comment_char(s)));
+}
+
+// ===========================================================================
+// Boolean
+
+TOML11_INLINE either boolean(const spec&)
+{
+    return either(literal("true"), literal("false"));
+}
+
+// ===========================================================================
+// Integer
+
+TOML11_INLINE digit::digit(const spec&) noexcept
+    : scanner_(char_type('0'), char_type('9'))
+{}
+
+TOML11_INLINE alpha::alpha(const spec&) noexcept
+    : scanner_(
+        character_in_range(char_type('a'), char_type('z')),
+        character_in_range(char_type('A'), char_type('Z'))
+    )
+{}
+
+TOML11_INLINE hexdig::hexdig(const spec& s) noexcept
+    : scanner_(
+        digit(s),
+        character_in_range(char_type('a'), char_type('f')),
+        character_in_range(char_type('A'), char_type('F'))
+    )
+{}
+
+// non-digit-graph = ([a-zA-Z]|unicode mb char)
+// graph           = ([a-zA-Z0-9]|unicode mb char)
+// suffix          = _ non-digit-graph (graph | _graph)
+TOML11_INLINE sequence num_suffix(const spec& s)
+{
+    const auto non_digit_graph = [&s]() {
+        return either(
+            alpha(s),
+            non_ascii(s)
+        );
+    };
+    const auto graph = [&s]() {
+        return either(
+            alpha(s),
+            digit(s),
+            non_ascii(s)
+        );
+    };
+
+    return sequence(
+            character(char_type('_')),
+            non_digit_graph(),
+            repeat_at_least(0,
+                either(
+                    sequence(character(char_type('_')), graph()),
+                    graph()
+                )
+            )
+        );
+}
+
+TOML11_INLINE sequence dec_int(const spec& s)
+{
+    const auto digit19 = []() {
+        return character_in_range(char_type('1'), char_type('9'));
+    };
+    return sequence(
+            maybe(character_either{char_type('-'), char_type('+')}),
+            either(
+                sequence(
+                    digit19(),
+                    repeat_at_least(1,
+                        either(
+                            digit(s),
+                            sequence(character(char_type('_')), digit(s))
+                        )
+                    )
+                ),
+                digit(s)
+            )
+        );
+}
+
+TOML11_INLINE sequence hex_int(const spec& s)
+{
+    return sequence(
+            literal("0x"),
+            hexdig(s),
+            repeat_at_least(0,
+                either(
+                    hexdig(s),
+                    sequence(character(char_type('_')), hexdig(s))
+                )
+            )
+        );
+}
+
+TOML11_INLINE sequence oct_int(const spec&)
+{
+    const auto digit07 = []() {
+        return character_in_range(char_type('0'), char_type('7'));
+    };
+    return sequence(
+            literal("0o"),
+            digit07(),
+            repeat_at_least(0,
+                either(
+                    digit07(),
+                    sequence(character(char_type('_')), digit07())
+                )
+            )
+        );
+}
+
+TOML11_INLINE sequence bin_int(const spec&)
+{
+    const auto digit01 = []() {
+        return character_either{char_type('0'), char_type('1')};
+    };
+    return sequence(
+            literal("0b"),
+            digit01(),
+            repeat_at_least(0,
+                either(
+                    digit01(),
+                    sequence(character(char_type('_')), digit01())
+                )
+            )
+        );
+}
+
+TOML11_INLINE either integer(const spec& s)
+{
+    return either(
+            hex_int(s),
+            oct_int(s),
+            bin_int(s),
+            dec_int(s)
+        );
+}
+
+
+// ===========================================================================
+// Floating
+
+TOML11_INLINE sequence zero_prefixable_int(const spec& s)
+{
+    return sequence(
+            digit(s),
+            repeat_at_least(0,
+                either(
+                    digit(s),
+                    sequence(character('_'), digit(s))
+                )
+            )
+        );
+}
+
+TOML11_INLINE sequence fractional_part(const spec& s)
+{
+    return sequence(
+            character('.'),
+            zero_prefixable_int(s)
+        );
+}
+
+TOML11_INLINE sequence exponent_part(const spec& s)
+{
+    return sequence(
+            character_either{char_type('e'), char_type('E')},
+            maybe(character_either{char_type('+'), char_type('-')}),
+            zero_prefixable_int(s)
+        );
+}
+
+TOML11_INLINE sequence hex_floating(const spec& s)
+{
+    // C99 hexfloat (%a)
+    // [+-]? 0x ( [0-9a-fA-F]*\.[0-9a-fA-F]+ | [0-9a-fA-F]+\.? ) [pP] [+-]? [0-9]+
+
+    // - 0x(int).(frac)p[+-](int)
+    // - 0x(int).p[+-](int)
+    // - 0x.(frac)p[+-](int)
+    // - 0x(int)p[+-](int)
+
+    return sequence(
+            maybe(character_either{char_type('+'), char_type('-')}),
+            character('0'),
+            character_either{char_type('x'), char_type('X')},
+            either(
+                sequence(
+                    repeat_at_least(0, hexdig(s)),
+                    character('.'),
+                    repeat_at_least(1, hexdig(s))
+                ),
+                sequence(
+                    repeat_at_least(1, hexdig(s)),
+                    maybe(character('.'))
+                )
+            ),
+            character_either{char_type('p'), char_type('P')},
+            maybe(character_either{char_type('+'), char_type('-')}),
+            repeat_at_least(1, character_in_range('0', '9'))
+        );
+}
+
+TOML11_INLINE either floating(const spec& s)
+{
+    return either(
+            sequence(
+                dec_int(s),
+                either(
+                    exponent_part(s),
+                    sequence(fractional_part(s), maybe(exponent_part(s)))
+                )
+            ),
+            sequence(
+                maybe(character_either{char_type('-'), char_type('+')}),
+                either(literal("inf"), literal("nan"))
+            )
+        );
+}
+
+// ===========================================================================
+// Datetime
+
+TOML11_INLINE sequence local_date(const spec& s)
+{
+    return sequence(
+            repeat_exact(4, digit(s)),
+            character('-'),
+            repeat_exact(2, digit(s)),
+            character('-'),
+            repeat_exact(2, digit(s))
+        );
+}
+TOML11_INLINE sequence local_time(const spec& s)
+{
+    auto time = sequence(
+            repeat_exact(2, digit(s)),
+            character(':'),
+            repeat_exact(2, digit(s))
+        );
+
+    if(s.v1_1_0_make_seconds_optional)
+    {
+        time.push_back(maybe(sequence(
+                character(':'),
+                repeat_exact(2, digit(s)),
+                maybe(sequence(character('.'), repeat_at_least(1, digit(s))))
+            )));
+    }
+    else
+    {
+        time.push_back(character(':'));
+        time.push_back(repeat_exact(2, digit(s)));
+        time.push_back(
+            maybe(sequence(character('.'), repeat_at_least(1, digit(s))))
+        );
+    }
+
+    return time;
+}
+TOML11_INLINE either time_offset(const spec& s)
+{
+    return either(
+            character_either{'Z', 'z'},
+            sequence(character_either{'+', '-'},
+                     repeat_exact(2, digit(s)),
+                     character(':'),
+                     repeat_exact(2, digit(s))
+             )
+        );
+}
+TOML11_INLINE sequence full_time(const spec& s)
+{
+    return sequence(local_time(s), time_offset(s));
+}
+TOML11_INLINE character_either time_delim(const spec&)
+{
+    return character_either{'T', 't', ' '};
+}
+TOML11_INLINE sequence local_datetime(const spec& s)
+{
+    return sequence(local_date(s), time_delim(s), local_time(s));
+}
+TOML11_INLINE sequence offset_datetime(const spec& s)
+{
+    return sequence(local_date(s), time_delim(s), full_time(s));
+}
+
+// ===========================================================================
+// String
+
+TOML11_INLINE sequence escaped(const spec& s)
+{
+    character_either escape_char{
+        '\"','\\', 'b', 'f', 'n', 'r', 't'
+    };
+    if(s.v1_1_0_add_escape_sequence_e)
+    {
+        escape_char.push_back(char_type('e'));
+    }
+
+    either escape_seq(
+            std::move(escape_char),
+            sequence(character('u'), repeat_exact(4, hexdig(s))),
+            sequence(character('U'), repeat_exact(8, hexdig(s)))
+        );
+
+    if(s.v1_1_0_add_escape_sequence_x)
+    {
+        escape_seq.push_back(
+            sequence(character('x'), repeat_exact(2, hexdig(s)))
+        );
+    }
+
+    return sequence(
+            character('\\'),
+            std::move(escape_seq)
+        );
+}
+
+TOML11_INLINE either basic_char(const spec& s)
+{
+    const auto basic_unescaped = [&s]() {
+        return either(
+                wschar(s),
+                character(0x21),                // 22 is "
+                character_in_range(0x23, 0x5B), // 5C is backslash
+                character_in_range(0x5D, 0x7E), // 7F is DEL
+                non_ascii(s)
+            );
+    };
+    return either(basic_unescaped(), escaped(s));
+}
+
+TOML11_INLINE sequence basic_string(const spec& s)
+{
+    return sequence(
+            character('"'),
+            repeat_at_least(0, basic_char(s)),
+            character('"')
+        );
+}
+
+// ---------------------------------------------------------------------------
+// multiline string
+
+TOML11_INLINE sequence escaped_newline(const spec& s)
+{
+    return sequence(
+            character('\\'), ws(s), newline(s),
+            repeat_at_least(0, either(wschar(s), newline(s)))
+        );
+}
+
+TOML11_INLINE sequence ml_basic_string(const spec& s)
+{
+    const auto mlb_content = [&s]() {
+        return either(basic_char(s), newline(s), escaped_newline(s));
+    };
+    const auto mlb_quotes = []() {
+        return either(literal("\"\""), character('\"'));
+    };
+
+    return sequence(
+            literal("\"\"\""),
+            maybe(newline(s)),
+            repeat_at_least(0, mlb_content()),
+            repeat_at_least(0,
+                sequence(
+                    mlb_quotes(),
+                    repeat_at_least(1, mlb_content())
+                )
+            ),
+            // XXX """ and mlb_quotes are intentionally reordered to avoid
+            //     unexpected match of mlb_quotes
+            literal("\"\"\""),
+            maybe(mlb_quotes())
+        );
+}
+
+// ---------------------------------------------------------------------------
+// literal string
+
+TOML11_INLINE either literal_char(const spec& s)
+{
+    return either(
+            character         (0x09),
+            character_in_range(0x20, 0x26),
+            character_in_range(0x28, 0x7E),
+            non_ascii(s)
+        );
+}
+
+TOML11_INLINE sequence literal_string(const spec& s)
+{
+    return sequence(
+            character('\''),
+            repeat_at_least(0, literal_char(s)),
+            character('\'')
+        );
+}
+
+TOML11_INLINE sequence ml_literal_string(const spec& s)
+{
+    const auto mll_quotes = []() {
+        return either(literal("''"), character('\''));
+    };
+    const auto mll_content = [&s]() {
+        return either(literal_char(s), newline(s));
+    };
+
+    return sequence(
+            literal("'''"),
+            maybe(newline(s)),
+            repeat_at_least(0, mll_content()),
+            repeat_at_least(0, sequence(
+                    mll_quotes(),
+                    repeat_at_least(1, mll_content())
+                )
+            ),
+            literal("'''"),
+            maybe(mll_quotes())
+            // XXX ''' and mll_quotes are intentionally reordered to avoid
+            //     unexpected match of mll_quotes
+        );
+}
+
+TOML11_INLINE either string(const spec& s)
+{
+    return either(
+            ml_basic_string(s),
+            ml_literal_string(s),
+            basic_string(s),
+            literal_string(s)
+        );
+}
+
+// ===========================================================================
+// Keys
+
+// to keep `expected_chars` simple
+TOML11_INLINE non_ascii_key_char::non_ascii_key_char(const spec& s) noexcept
+{
+    assert(s.v1_1_0_allow_non_english_in_bare_keys);
+    (void)s; // for NDEBUG
+}
+
+TOML11_INLINE std::uint32_t non_ascii_key_char::read_utf8(location& loc) const
+{
+    // U+0000   ... U+0079  ; 0xxx_xxxx
+    // U+0080   ... U+07FF  ; 110y_yyyx 10xx_xxxx;
+    // U+0800   ... U+FFFF  ; 1110_yyyy 10yx_xxxx 10xx_xxxx
+    // U+010000 ... U+10FFFF; 1111_0yyy 10yy_xxxx 10xx_xxxx 10xx_xxxx
+
+    const unsigned char b1 = loc.current(); loc.advance(1);
+    if(b1 < 0x80)
+    {
+        return static_cast<std::uint32_t>(b1);
+    }
+    else if((b1 >> 5) == 6) // 0b110 == 6
+    {
+        const auto b2 = loc.current(); loc.advance(1);
+
+        const std::uint32_t c1 = b1 & ((1 << 5) - 1);
+        const std::uint32_t c2 = b2 & ((1 << 6) - 1);
+        const std::uint32_t codep = (c1 << 6) + c2;
+
+        if(codep < 0x80)
+        {
+            return 0xFFFFFFFF;
+        }
+        return codep;
+    }
+    else if((b1 >> 4) == 14) // 0b1110 == 14
+    {
+        const auto b2 = loc.current(); loc.advance(1); if(loc.eof()) {return 0xFFFFFFFF;}
+        const auto b3 = loc.current(); loc.advance(1);
+
+        const std::uint32_t c1 = b1 & ((1 << 4) - 1);
+        const std::uint32_t c2 = b2 & ((1 << 6) - 1);
+        const std::uint32_t c3 = b3 & ((1 << 6) - 1);
+
+        const std::uint32_t codep = (c1 << 12) + (c2 << 6) + c3;
+        if(codep < 0x800)
+        {
+            return 0xFFFFFFFF;
+        }
+        return codep;
+    }
+    else if((b1 >> 3) == 30) // 0b11110 == 30
+    {
+        const auto b2 = loc.current(); loc.advance(1); if(loc.eof()) {return 0xFFFFFFFF;}
+        const auto b3 = loc.current(); loc.advance(1); if(loc.eof()) {return 0xFFFFFFFF;}
+        const auto b4 = loc.current(); loc.advance(1);
+
+        const std::uint32_t c1 = b1 & ((1 << 3) - 1);
+        const std::uint32_t c2 = b2 & ((1 << 6) - 1);
+        const std::uint32_t c3 = b3 & ((1 << 6) - 1);
+        const std::uint32_t c4 = b4 & ((1 << 6) - 1);
+        const std::uint32_t codep = (c1 << 18) + (c2 << 12) + (c3 << 6) + c4;
+
+        if(codep < 0x10000)
+        {
+            return 0xFFFFFFFF;
+        }
+        return codep;
+    }
+    else // not a Unicode codepoint in UTF-8
+    {
+        return 0xFFFFFFFF;
+    }
+}
+
+TOML11_INLINE region non_ascii_key_char::scan(location& loc) const
+{
+    if(loc.eof()) {return region{};}
+
+    const auto first = loc;
+
+    const auto cp = read_utf8(loc);
+
+    if(cp == 0xFFFFFFFF)
+    {
+        return region{};
+    }
+
+    // ALPHA / DIGIT / %x2D / %x5F    ; a-z A-Z 0-9 - _
+    // / %xB2 / %xB3 / %xB9 / %xBC-BE ; superscript digits, fractions
+    // / %xC0-D6 / %xD8-F6 / %xF8-37D ; non-symbol chars in Latin block
+    // / %x37F-1FFF                   ; exclude GREEK QUESTION MARK, which is basically a semi-colon
+    // / %x200C-200D / %x203F-2040    ; from General Punctuation Block, include the two tie symbols and ZWNJ, ZWJ
+    // / %x2070-218F / %x2460-24FF    ; include super-/subscripts, letterlike/numberlike forms, enclosed alphanumerics
+    // / %x2C00-2FEF / %x3001-D7FF    ; skip arrows, math, box drawing etc, skip 2FF0-3000 ideographic up/down markers and spaces
+    // / %xF900-FDCF / %xFDF0-FFFD    ; skip D800-DFFF surrogate block, E000-F8FF Private Use area, FDD0-FDEF intended for process-internal use (unicode)
+    // / %x10000-EFFFF                ; all chars outside BMP range, excluding Private Use planes (F0000-10FFFF)
+
+    if(cp == 0xB2 || cp == 0xB3 || cp == 0xB9 || (0xBC <= cp && cp <= 0xBE) ||
+       (0xC0    <= cp && cp <= 0xD6  ) || (0xD8 <= cp && cp <= 0xF6) || (0xF8 <= cp && cp <= 0x37D) ||
+       (0x37F   <= cp && cp <= 0x1FFF) ||
+       (0x200C  <= cp && cp <= 0x200D) || (0x203F <= cp && cp <= 0x2040) ||
+       (0x2070  <= cp && cp <= 0x218F) || (0x2460 <= cp && cp <= 0x24FF) ||
+       (0x2C00  <= cp && cp <= 0x2FEF) || (0x3001 <= cp && cp <= 0xD7FF) ||
+       (0xF900  <= cp && cp <= 0xFDCF) || (0xFDF0 <= cp && cp <= 0xFFFD) ||
+       (0x10000 <= cp && cp <= 0xEFFFF) )
+    {
+        return region(first, loc);
+    }
+    loc = first;
+    return region{};
+}
+
+TOML11_INLINE repeat_at_least unquoted_key(const spec& s)
+{
+    auto keychar = either(
+            alpha(s), digit(s), character{0x2D}, character{0x5F}
+        );
+
+    if(s.v1_1_0_allow_non_english_in_bare_keys)
+    {
+        keychar.push_back(non_ascii_key_char(s));
+    }
+
+    return repeat_at_least(1, std::move(keychar));
+}
+
+TOML11_INLINE either quoted_key(const spec& s)
+{
+    return either(basic_string(s), literal_string(s));
+}
+
+TOML11_INLINE either simple_key(const spec& s)
+{
+    return either(unquoted_key(s), quoted_key(s));
+}
+
+TOML11_INLINE sequence dot_sep(const spec& s)
+{
+    return sequence(ws(s), character('.'), ws(s));
+}
+
+TOML11_INLINE sequence dotted_key(const spec& s)
+{
+    return sequence(
+        simple_key(s),
+        repeat_at_least(1, sequence(dot_sep(s), simple_key(s)))
+    );
+}
+
+TOML11_INLINE key::key(const spec& s) noexcept
+    : scanner_(dotted_key(s), simple_key(s))
+{}
+
+TOML11_INLINE sequence keyval_sep(const spec& s)
+{
+    return sequence(ws(s), character('='), ws(s));
+}
+
+// ===========================================================================
+// Table key
+
+TOML11_INLINE sequence std_table(const spec& s)
+{
+    return sequence(character('['), ws(s), key(s), ws(s), character(']'));
+}
+
+TOML11_INLINE sequence array_table(const spec& s)
+{
+    return sequence(literal("[["), ws(s), key(s), ws(s), literal("]]"));
+}
+
+// ===========================================================================
+// extension: null
+
+TOML11_INLINE literal null_value(const spec&)
+{
+    return literal("null");
+}
+
+} // namespace syntax
+} // namespace detail
+} // namespace toml
+#endif // TOML11_SYNTAX_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/impl/value_t_impl.hpp b/src/frontend/qt_sdl/toml/toml11/impl/value_t_impl.hpp
new file mode 100644
index 00000000..784dc8b2
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/impl/value_t_impl.hpp
@@ -0,0 +1,40 @@
+#ifndef TOML11_VALUE_T_IMPL_HPP
+#define TOML11_VALUE_T_IMPL_HPP
+
+#include "../fwd/value_t_fwd.hpp"
+
+#include <ostream>
+#include <sstream>
+#include <string>
+
+namespace toml
+{
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, value_t t)
+{
+    switch(t)
+    {
+        case value_t::boolean         : os << "boolean";         return os;
+        case value_t::integer         : os << "integer";         return os;
+        case value_t::floating        : os << "floating";        return os;
+        case value_t::string          : os << "string";          return os;
+        case value_t::offset_datetime : os << "offset_datetime"; return os;
+        case value_t::local_datetime  : os << "local_datetime";  return os;
+        case value_t::local_date      : os << "local_date";      return os;
+        case value_t::local_time      : os << "local_time";      return os;
+        case value_t::array           : os << "array";           return os;
+        case value_t::table           : os << "table";           return os;
+        case value_t::empty           : os << "empty";           return os;
+        default                       : os << "unknown";         return os;
+    }
+}
+
+TOML11_INLINE std::string to_string(value_t t)
+{
+    std::ostringstream oss;
+    oss << t;
+    return oss.str();
+}
+
+} // namespace toml
+#endif // TOML11_VALUE_T_IMPL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml/into.hpp b/src/frontend/qt_sdl/toml/toml11/into.hpp
similarity index 79%
rename from src/frontend/qt_sdl/toml/toml/into.hpp
rename to src/frontend/qt_sdl/toml/toml11/into.hpp
index 74495560..86a0020e 100644
--- a/src/frontend/qt_sdl/toml/toml/into.hpp
+++ b/src/frontend/qt_sdl/toml/toml11/into.hpp
@@ -1,5 +1,3 @@
-//     Copyright Toru Niina 2019.
-// Distributed under the MIT License.
 #ifndef TOML11_INTO_HPP
 #define TOML11_INTO_HPP
 
diff --git a/src/frontend/qt_sdl/toml/toml11/literal.hpp b/src/frontend/qt_sdl/toml/toml11/literal.hpp
new file mode 100644
index 00000000..e30e7a83
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/literal.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_LITERAL_HPP
+#define TOML11_LITERAL_HPP
+
+#include "fwd/literal_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/literal_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_LITERAL_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/location.hpp b/src/frontend/qt_sdl/toml/toml11/location.hpp
new file mode 100644
index 00000000..fd232744
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/location.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_LOCATION_HPP
+#define TOML11_LOCATION_HPP
+
+#include "fwd/location_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/location_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_LOCATION_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/ordered_map.hpp b/src/frontend/qt_sdl/toml/toml11/ordered_map.hpp
new file mode 100644
index 00000000..b9cd3042
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/ordered_map.hpp
@@ -0,0 +1,265 @@
+#ifndef TOML11_ORDERED_MAP_HPP
+#define TOML11_ORDERED_MAP_HPP
+
+#include <algorithm>
+#include <stdexcept>
+#include <utility>
+#include <vector>
+
+namespace toml
+{
+
+namespace detail
+{
+template<typename Cmp>
+struct ordered_map_ebo_container
+{
+    Cmp cmp_; // empty base optimization for empty Cmp type
+};
+} // detail
+
+template<typename Key, typename Val, typename Cmp = std::equal_to<Key>,
+         typename Allocator = std::allocator<std::pair<Key, Val>>>
+class ordered_map : detail::ordered_map_ebo_container<Cmp>
+{
+  public:
+    using key_type    = Key;
+    using mapped_type = Val;
+    using value_type  = std::pair<Key, Val>;
+
+    using key_compare    = Cmp;
+    using allocator_type = Allocator;
+
+    using container_type  = std::vector<value_type, Allocator>;
+    using reference       = typename container_type::reference;
+    using pointer         = typename container_type::pointer;
+    using const_reference = typename container_type::const_reference;
+    using const_pointer   = typename container_type::const_pointer;
+    using iterator        = typename container_type::iterator;
+    using const_iterator  = typename container_type::const_iterator;
+    using size_type       = typename container_type::size_type;
+    using difference_type = typename container_type::difference_type;
+
+  private:
+
+    using ebo_base = detail::ordered_map_ebo_container<Cmp>;
+
+  public:
+
+    ordered_map() = default;
+    ~ordered_map() = default;
+    ordered_map(const ordered_map&) = default;
+    ordered_map(ordered_map&&)      = default;
+    ordered_map& operator=(const ordered_map&) = default;
+    ordered_map& operator=(ordered_map&&)      = default;
+
+    ordered_map(const ordered_map& other, const Allocator& alloc)
+        : container_(other.container_, alloc)
+    {}
+    ordered_map(ordered_map&& other, const Allocator& alloc)
+        : container_(std::move(other.container_), alloc)
+    {}
+
+    explicit ordered_map(const Cmp& cmp, const Allocator& alloc = Allocator())
+        : ebo_base{cmp}, container_(alloc)
+    {}
+    explicit ordered_map(const Allocator& alloc)
+        : container_(alloc)
+    {}
+
+    template<typename InputIterator>
+    ordered_map(InputIterator first, InputIterator last, const Cmp& cmp = Cmp(), const Allocator& alloc = Allocator())
+        : ebo_base{cmp}, container_(first, last, alloc)
+    {}
+    template<typename InputIterator>
+    ordered_map(InputIterator first, InputIterator last, const Allocator& alloc)
+        : container_(first, last, alloc)
+    {}
+
+    ordered_map(std::initializer_list<value_type> v, const Cmp& cmp = Cmp(), const Allocator& alloc = Allocator())
+        : ebo_base{cmp}, container_(std::move(v), alloc)
+    {}
+    ordered_map(std::initializer_list<value_type> v, const Allocator& alloc)
+        : container_(std::move(v), alloc)
+    {}
+    ordered_map& operator=(std::initializer_list<value_type> v)
+    {
+        this->container_ = std::move(v);
+        return *this;
+    }
+
+    iterator       begin()        noexcept {return container_.begin();}
+    iterator       end()          noexcept {return container_.end();}
+    const_iterator begin()  const noexcept {return container_.begin();}
+    const_iterator end()    const noexcept {return container_.end();}
+    const_iterator cbegin() const noexcept {return container_.cbegin();}
+    const_iterator cend()   const noexcept {return container_.cend();}
+
+    bool        empty()    const noexcept {return container_.empty();}
+    std::size_t size()     const noexcept {return container_.size();}
+    std::size_t max_size() const noexcept {return container_.max_size();}
+
+    void clear() {container_.clear();}
+
+    void push_back(const value_type& v)
+    {
+        if(this->contains(v.first))
+        {
+            throw std::out_of_range("ordered_map: value already exists");
+        }
+        container_.push_back(v);
+    }
+    void push_back(value_type&& v)
+    {
+        if(this->contains(v.first))
+        {
+            throw std::out_of_range("ordered_map: value already exists");
+        }
+        container_.push_back(std::move(v));
+    }
+    void emplace_back(key_type k, mapped_type v)
+    {
+        if(this->contains(k))
+        {
+            throw std::out_of_range("ordered_map: value already exists");
+        }
+        container_.emplace_back(std::move(k), std::move(v));
+    }
+    void pop_back()  {container_.pop_back();}
+
+    void insert(value_type kv)
+    {
+        if(this->contains(kv.first))
+        {
+            throw std::out_of_range("ordered_map: value already exists");
+        }
+        container_.push_back(std::move(kv));
+    }
+    void emplace(key_type k, mapped_type v)
+    {
+        if(this->contains(k))
+        {
+            throw std::out_of_range("ordered_map: value already exists");
+        }
+        container_.emplace_back(std::move(k), std::move(v));
+    }
+
+    std::size_t count(const key_type& key) const
+    {
+        if(this->find(key) != this->end())
+        {
+            return 1;
+        }
+        else
+        {
+            return 0;
+        }
+    }
+    bool contains(const key_type& key) const
+    {
+        return this->find(key) != this->end();
+    }
+    iterator find(const key_type& key) noexcept
+    {
+        return std::find_if(this->begin(), this->end(),
+            [&key, this](const value_type& v) {return this->cmp_(v.first, key);});
+    }
+    const_iterator find(const key_type& key) const noexcept
+    {
+        return std::find_if(this->begin(), this->end(),
+            [&key, this](const value_type& v) {return this->cmp_(v.first, key);});
+    }
+
+    mapped_type&       at(const key_type& k)
+    {
+        const auto iter = this->find(k);
+        if(iter == this->end())
+        {
+            throw std::out_of_range("ordered_map: no such element");
+        }
+        return iter->second;
+    }
+    mapped_type const& at(const key_type& k) const
+    {
+        const auto iter = this->find(k);
+        if(iter == this->end())
+        {
+            throw std::out_of_range("ordered_map: no such element");
+        }
+        return iter->second;
+    }
+
+    mapped_type& operator[](const key_type& k)
+    {
+        const auto iter = this->find(k);
+        if(iter == this->end())
+        {
+            this->container_.emplace_back(k, mapped_type{});
+            return this->container_.back().second;
+        }
+        return iter->second;
+    }
+
+    mapped_type const& operator[](const key_type& k) const
+    {
+        const auto iter = this->find(k);
+        if(iter == this->end())
+        {
+            throw std::out_of_range("ordered_map: no such element");
+        }
+        return iter->second;
+    }
+
+    key_compare key_comp() const {return this->cmp_;}
+
+    void swap(ordered_map& other)
+    {
+        container_.swap(other.container_);
+    }
+
+  private:
+
+    container_type container_;
+};
+
+template<typename K, typename V, typename C, typename A>
+bool operator==(const ordered_map<K,V,C,A>& lhs, const ordered_map<K,V,C,A>& rhs)
+{
+    return lhs.size() == rhs.size() && std::equal(lhs.begin(), lhs.end(), rhs.begin());
+}
+template<typename K, typename V, typename C, typename A>
+bool operator!=(const ordered_map<K,V,C,A>& lhs, const ordered_map<K,V,C,A>& rhs)
+{
+    return !(lhs == rhs);
+}
+template<typename K, typename V, typename C, typename A>
+bool operator<(const ordered_map<K,V,C,A>& lhs, const ordered_map<K,V,C,A>& rhs)
+{
+    return std::lexicographical_compare(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
+}
+template<typename K, typename V, typename C, typename A>
+bool operator>(const ordered_map<K,V,C,A>& lhs, const ordered_map<K,V,C,A>& rhs)
+{
+    return rhs < lhs;
+}
+template<typename K, typename V, typename C, typename A>
+bool operator<=(const ordered_map<K,V,C,A>& lhs, const ordered_map<K,V,C,A>& rhs)
+{
+    return !(lhs > rhs);
+}
+template<typename K, typename V, typename C, typename A>
+bool operator>=(const ordered_map<K,V,C,A>& lhs, const ordered_map<K,V,C,A>& rhs)
+{
+    return !(lhs < rhs);
+}
+
+template<typename K, typename V, typename C, typename A>
+void swap(ordered_map<K,V,C,A>& lhs, ordered_map<K,V,C,A>& rhs)
+{
+    lhs.swap(rhs);
+    return;
+}
+
+
+} // toml
+#endif // TOML11_ORDERED_MAP_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/parser.hpp b/src/frontend/qt_sdl/toml/toml11/parser.hpp
new file mode 100644
index 00000000..6d147311
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/parser.hpp
@@ -0,0 +1,3829 @@
+#ifndef TOML11_PARSER_HPP
+#define TOML11_PARSER_HPP
+
+#include "context.hpp"
+#include "datetime.hpp"
+#include "error_info.hpp"
+#include "region.hpp"
+#include "result.hpp"
+#include "scanner.hpp"
+#include "skip.hpp"
+#include "syntax.hpp"
+#include "value.hpp"
+
+#include <fstream>
+#include <sstream>
+
+#include <cassert>
+#include <cmath>
+
+#if defined(TOML11_HAS_FILESYSTEM) && TOML11_HAS_FILESYSTEM
+#include <filesystem>
+#endif
+
+namespace toml
+{
+
+struct syntax_error final : public ::toml::exception
+{
+  public:
+    syntax_error(std::string what_arg, std::vector<error_info> err)
+        : what_(std::move(what_arg)), err_(std::move(err))
+    {}
+    ~syntax_error() noexcept override = default;
+
+    const char* what() const noexcept override {return what_.c_str();}
+
+    std::vector<error_info> const& errors() const noexcept
+    {
+        return err_;
+    }
+
+  private:
+    std::string what_;
+    std::vector<error_info> err_;
+};
+
+struct file_io_error final : public ::toml::exception
+{
+  public:
+
+    file_io_error(const std::string& msg, const std::string& fname)
+        : errno_(cxx::make_nullopt()),
+          what_(msg + " \"" + fname + "\"")
+    {}
+    file_io_error(int errnum, const std::string& msg, const std::string& fname)
+        : errno_(errnum),
+          what_(msg + " \"" + fname + "\": errno=" + std::to_string(errnum))
+    {}
+    ~file_io_error() noexcept override = default;
+
+    const char* what() const noexcept override {return what_.c_str();}
+
+    bool has_errno() const noexcept {return errno_.has_value();}
+    int  get_errno() const noexcept {return errno_.value_or(0);}
+
+  private:
+
+    cxx::optional<int> errno_;
+    std::string what_;
+};
+
+namespace detail
+{
+
+/* ============================================================================
+ *    __ ___ _ __  _ __  ___ _ _
+ *   / _/ _ \ '  \| '  \/ _ \ ' \
+ *   \__\___/_|_|_|_|_|_\___/_||_|
+ */
+
+template<typename S>
+error_info make_syntax_error(std::string title,
+        const S& scanner, location loc, std::string suffix = "")
+{
+    auto msg = std::string("expected ") + scanner.expected_chars(loc);
+    auto src = source_location(region(loc));
+    return make_error_info(
+        std::move(title), std::move(src), std::move(msg), std::move(suffix));
+}
+
+
+/* ============================================================================
+ *                             _
+ *  __ ___ _ __  _ __  ___ _ _| |_
+ * / _/ _ \ '  \| '  \/ -_) ' \  _|
+ * \__\___/_|_|_|_|_|_\___|_||_\__|
+ */
+
+template<typename TC>
+result<cxx::optional<std::string>, error_info>
+parse_comment_line(location& loc, context<TC>& ctx)
+{
+    const auto& spec = ctx.toml_spec();
+    const auto first = loc;
+
+    skip_whitespace(loc, ctx);
+
+    const auto com_reg = syntax::comment(spec).scan(loc);
+    if(com_reg.is_ok())
+    {
+        // once comment started, newline must follow (or reach EOF).
+        if( ! loc.eof() && ! syntax::newline(spec).scan(loc).is_ok())
+        {
+            while( ! loc.eof()) // skip until newline to continue parsing
+            {
+                loc.advance();
+                if(loc.current() == '\n') { /*skip LF*/ loc.advance(); break; }
+            }
+            return err(make_error_info("toml::parse_comment_line: "
+                "newline (LF / CRLF) or EOF is expected",
+                source_location(region(loc)), "but got this",
+                "Hint: most of the control characters are not allowed in comments"));
+        }
+        return ok(cxx::optional<std::string>(com_reg.as_string()));
+    }
+    else
+    {
+        loc = first; // rollback whitespace to parse indent
+        return ok(cxx::optional<std::string>(cxx::make_nullopt()));
+    }
+}
+
+/* ============================================================================
+ *  ___           _
+ * | _ ) ___  ___| |___ __ _ _ _
+ * | _ \/ _ \/ _ \ / -_) _` | ' \
+ * |___/\___/\___/_\___\__,_|_||_|
+ */
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_boolean(location& loc, const context<TC>& ctx)
+{
+    const auto& spec = ctx.toml_spec();
+
+    // ----------------------------------------------------------------------
+    // check syntax
+    auto reg = syntax::boolean(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_boolean: "
+            "invalid boolean: boolean must be `true` or `false`, in lowercase. "
+            "string must be surrounded by `\"`", syntax::boolean(spec), loc));
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+    const auto str = reg.as_string();
+    const auto val = [&str]() {
+        if(str == "true")
+        {
+            return true;
+        }
+        else
+        {
+            assert(str == "false");
+            return false;
+        }
+    }();
+
+    // ----------------------------------------------------------------------
+    // no format info for boolean
+    boolean_format_info fmt;
+
+    return ok(basic_value<TC>(val, std::move(fmt), {}, std::move(reg)));
+}
+
+/* ============================================================================
+ *  ___     _
+ * |_ _|_ _| |_ ___ __ _ ___ _ _
+ *  | || ' \  _/ -_) _` / -_) '_|
+ * |___|_||_\__\___\__, \___|_|
+ *                 |___/
+ */
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_bin_integer(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+    auto reg = syntax::bin_int(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_bin_integer: "
+            "invalid integer: bin_int must be like: 0b0101, 0b1111_0000",
+            syntax::bin_int(spec), loc));
+    }
+
+    auto str = reg.as_string();
+
+    integer_format_info fmt;
+    fmt.fmt   = integer_format::bin;
+    fmt.width = str.size() - 2 - static_cast<std::size_t>(std::count(str.begin(), str.end(), '_'));
+
+    const auto first_underscore = std::find(str.rbegin(), str.rend(), '_');
+    if(first_underscore != str.rend())
+    {
+        fmt.spacer = static_cast<std::size_t>(std::distance(str.rbegin(), first_underscore));
+    }
+
+    // skip prefix `0b` and zeros and underscores at the MSB
+    str.erase(str.begin(), std::find(std::next(str.begin(), 2), str.end(), '1'));
+
+    // remove all `_` before calling TC::parse_int
+    str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
+
+    // 0b0000_0000 becomes empty.
+    if(str.empty()) { str = "0"; }
+
+    const auto val = TC::parse_int(str, source_location(region(loc)), 2);
+    if(val.is_ok())
+    {
+        return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, std::move(reg)));
+    }
+    else
+    {
+        loc = first;
+        return err(val.as_err());
+    }
+}
+
+// ----------------------------------------------------------------------------
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_oct_integer(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+    auto reg = syntax::oct_int(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_oct_integer: "
+            "invalid integer: oct_int must be like: 0o775, 0o04_44",
+            syntax::oct_int(spec), loc));
+    }
+
+    auto str = reg.as_string();
+
+    integer_format_info fmt;
+    fmt.fmt   = integer_format::oct;
+    fmt.width = str.size() - 2 - static_cast<std::size_t>(std::count(str.begin(), str.end(), '_'));
+
+    const auto first_underscore = std::find(str.rbegin(), str.rend(), '_');
+    if(first_underscore != str.rend())
+    {
+        fmt.spacer = static_cast<std::size_t>(std::distance(str.rbegin(), first_underscore));
+    }
+
+    // skip prefix `0o` and zeros and underscores at the MSB
+    str.erase(str.begin(), std::find_if(
+                std::next(str.begin(), 2), str.end(), [](const char c) {
+                    return c != '0' && c != '_';
+                }));
+
+    // remove all `_` before calling TC::parse_int
+    str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
+
+    // 0o0000_0000 becomes empty.
+    if(str.empty()) { str = "0"; }
+
+    const auto val = TC::parse_int(str, source_location(region(loc)), 8);
+    if(val.is_ok())
+    {
+        return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, std::move(reg)));
+    }
+    else
+    {
+        loc = first;
+        return err(val.as_err());
+    }
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_hex_integer(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+    auto reg = syntax::hex_int(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_hex_integer: "
+            "invalid integer: hex_int must be like: 0xC0FFEE, 0xdead_beef",
+            syntax::hex_int(spec), loc));
+    }
+
+    auto str = reg.as_string();
+
+    integer_format_info fmt;
+    fmt.fmt   = integer_format::hex;
+    fmt.width = str.size() - 2 - static_cast<std::size_t>(std::count(str.begin(), str.end(), '_'));
+
+    const auto first_underscore = std::find(str.rbegin(), str.rend(), '_');
+    if(first_underscore != str.rend())
+    {
+        fmt.spacer = static_cast<std::size_t>(std::distance(str.rbegin(), first_underscore));
+    }
+
+    // skip prefix `0x` and zeros and underscores at the MSB
+    str.erase(str.begin(), std::find_if(
+                std::next(str.begin(), 2), str.end(), [](const char c) {
+                    return c != '0' && c != '_';
+                }));
+
+    // remove all `_` before calling TC::parse_int
+    str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
+
+    // 0x0000_0000 becomes empty.
+    if(str.empty()) { str = "0"; }
+
+    // prefix zero and _ is removed. check if it uses upper/lower case.
+    // if both upper and lower case letters are found, set upper=true.
+    const auto lower_not_found = std::find_if(str.begin(), str.end(),
+        [](const char c) { return std::islower(static_cast<int>(c)) != 0; }) == str.end();
+    const auto upper_found = std::find_if(str.begin(), str.end(),
+        [](const char c) { return std::isupper(static_cast<int>(c)) != 0; }) != str.end();
+    fmt.uppercase = lower_not_found || upper_found;
+
+    const auto val = TC::parse_int(str, source_location(region(loc)), 16);
+    if(val.is_ok())
+    {
+        return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, std::move(reg)));
+    }
+    else
+    {
+        loc = first;
+        return err(val.as_err());
+    }
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_dec_integer(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    // ----------------------------------------------------------------------
+    // check syntax
+    auto reg = syntax::dec_int(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_dec_integer: "
+            "invalid integer: dec_int must be like: 42, 123_456_789",
+            syntax::dec_int(spec), loc));
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+    auto str = reg.as_string();
+
+    integer_format_info fmt;
+    fmt.fmt = integer_format::dec;
+    fmt.width = str.size() - static_cast<std::size_t>(std::count(str.begin(), str.end(), '_'));
+
+    const auto first_underscore = std::find(str.rbegin(), str.rend(), '_');
+    if(first_underscore != str.rend())
+    {
+        fmt.spacer = static_cast<std::size_t>(std::distance(str.rbegin(), first_underscore));
+    }
+
+    // remove all `_` before calling TC::parse_int
+    str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
+
+    auto src = source_location(region(loc));
+    const auto val = TC::parse_int(str, src, 10);
+    if(val.is_err())
+    {
+        loc = first;
+        return err(val.as_err());
+    }
+
+    // ----------------------------------------------------------------------
+    // parse suffix (extension)
+
+    if(spec.ext_num_suffix && loc.current() == '_')
+    {
+        const auto sfx_reg = syntax::num_suffix(spec).scan(loc);
+        if( ! sfx_reg.is_ok())
+        {
+            loc = first;
+            return err(make_error_info("toml::parse_dec_integer: "
+                "invalid suffix: should be `_ non-digit-graph (graph | _graph)`",
+                source_location(region(loc)), "here"));
+        }
+        auto sfx = sfx_reg.as_string();
+        assert( ! sfx.empty() && sfx.front() == '_');
+        sfx.erase(sfx.begin()); // remove the first `_`
+
+        fmt.suffix = sfx;
+    }
+
+    return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, std::move(reg)));
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_integer(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+
+    if( ! loc.eof() && (loc.current() == '+' || loc.current() == '-'))
+    {
+        // skip +/- to diagnose +0xDEADBEEF or -0b0011 (invalid).
+        // without this, +0xDEAD_BEEF will be parsed as a decimal int and
+        // unexpected "xDEAD_BEEF" will appear after integer "+0".
+        loc.advance();
+    }
+
+    if( ! loc.eof() && loc.current() == '0')
+    {
+        loc.advance();
+        if(loc.eof())
+        {
+            // `[+-]?0`. parse as an decimal integer.
+            loc = first;
+            return parse_dec_integer(loc, ctx);
+        }
+
+        const auto prefix = loc.current();
+        auto prefix_src = source_location(region(loc));
+
+        loc = first;
+
+        if(prefix == 'b') {return parse_bin_integer(loc, ctx);}
+        if(prefix == 'o') {return parse_oct_integer(loc, ctx);}
+        if(prefix == 'x') {return parse_hex_integer(loc, ctx);}
+
+        if(std::isdigit(prefix))
+        {
+            auto src = source_location(region(loc));
+            return err(make_error_info("toml::parse_integer: "
+                "leading zero in an decimal integer is not allowed",
+                std::move(src), "leading zero"));
+        }
+    }
+
+    loc = first;
+    return parse_dec_integer(loc, ctx);
+}
+
+/* ============================================================================
+ *  ___ _           _   _
+ * | __| |___  __ _| |_(_)_ _  __ _
+ * | _|| / _ \/ _` |  _| | ' \/ _` |
+ * |_| |_\___/\__,_|\__|_|_||_\__, |
+ *                            |___/
+ */
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_floating(location& loc, const context<TC>& ctx)
+{
+    using floating_type = typename basic_value<TC>::floating_type;
+
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    // ----------------------------------------------------------------------
+    // check syntax
+    bool is_hex = false;
+    std::string str;
+    region reg;
+    if(spec.ext_hex_float && sequence(character('0'), character('x')).scan(loc).is_ok())
+    {
+        loc = first;
+        is_hex = true;
+
+        reg = syntax::hex_floating(spec).scan(loc);
+        if( ! reg.is_ok())
+        {
+            return err(make_syntax_error("toml::parse_floating: "
+                "invalid hex floating: float must be like: 0xABCp-3f",
+                syntax::floating(spec), loc));
+        }
+        str = reg.as_string();
+    }
+    else
+    {
+        reg = syntax::floating(spec).scan(loc);
+        if( ! reg.is_ok())
+        {
+            return err(make_syntax_error("toml::parse_floating: "
+                "invalid floating: float must be like: -3.14159_26535, 6.022e+23, "
+                "inf, or nan (lowercase).", syntax::floating(spec), loc));
+        }
+        str = reg.as_string();
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+
+    floating_format_info fmt;
+
+    if(is_hex)
+    {
+        fmt.fmt = floating_format::hex;
+    }
+    else
+    {
+        // since we already checked that the string conforms the TOML standard.
+        if(std::find(str.begin(), str.end(), 'e') != str.end() ||
+           std::find(str.begin(), str.end(), 'E') != str.end())
+        {
+            fmt.fmt = floating_format::scientific; // use exponent part
+        }
+        else
+        {
+            fmt.fmt = floating_format::fixed; // do not use exponent part
+        }
+    }
+
+    str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
+
+    floating_type val{0};
+
+    if(str == "inf" || str == "+inf")
+    {
+        TOML11_CONSTEXPR_IF(std::numeric_limits<floating_type>::has_infinity)
+        {
+            val = std::numeric_limits<floating_type>::infinity();
+        }
+        else
+        {
+            return err(make_error_info("toml::parse_floating: inf value found"
+                " but the current environment does not support inf. Please"
+                " make sure that the floating-point implementation conforms"
+                " IEEE 754/ISO 60559 international standard.",
+                source_location(region(loc)),
+                "floating_type: inf is not supported"));
+        }
+    }
+    else if(str == "-inf")
+    {
+        TOML11_CONSTEXPR_IF(std::numeric_limits<floating_type>::has_infinity)
+        {
+            val = -std::numeric_limits<floating_type>::infinity();
+        }
+        else
+        {
+            return err(make_error_info("toml::parse_floating: inf value found"
+                " but the current environment does not support inf. Please"
+                " make sure that the floating-point implementation conforms"
+                " IEEE 754/ISO 60559 international standard.",
+                source_location(region(loc)),
+                "floating_type: inf is not supported"));
+        }
+    }
+    else if(str == "nan" || str == "+nan")
+    {
+        TOML11_CONSTEXPR_IF(std::numeric_limits<floating_type>::has_quiet_NaN)
+        {
+            val = std::numeric_limits<floating_type>::quiet_NaN();
+        }
+        else TOML11_CONSTEXPR_IF(std::numeric_limits<floating_type>::has_signaling_NaN)
+        {
+            val = std::numeric_limits<floating_type>::signaling_NaN();
+        }
+        else
+        {
+            return err(make_error_info("toml::parse_floating: NaN value found"
+                " but the current environment does not support NaN. Please"
+                " make sure that the floating-point implementation conforms"
+                " IEEE 754/ISO 60559 international standard.",
+                source_location(region(loc)),
+                "floating_type: NaN is not supported"));
+        }
+    }
+    else if(str == "-nan")
+    {
+        using std::copysign;
+        TOML11_CONSTEXPR_IF(std::numeric_limits<floating_type>::has_quiet_NaN)
+        {
+            val = copysign(std::numeric_limits<floating_type>::quiet_NaN(), floating_type(-1));
+        }
+        else TOML11_CONSTEXPR_IF(std::numeric_limits<floating_type>::has_signaling_NaN)
+        {
+            val = copysign(std::numeric_limits<floating_type>::signaling_NaN(), floating_type(-1));
+        }
+        else
+        {
+            return err(make_error_info("toml::parse_floating: NaN value found"
+                " but the current environment does not support NaN. Please"
+                " make sure that the floating-point implementation conforms"
+                " IEEE 754/ISO 60559 international standard.",
+                source_location(region(loc)),
+                "floating_type: NaN is not supported"));
+        }
+    }
+    else
+    {
+        // set precision
+        const auto has_sign = ! str.empty() && (str.front() == '+' || str.front() == '-');
+        const auto decpoint = std::find(str.begin(), str.end(), '.');
+        const auto exponent = std::find_if(str.begin(), str.end(),
+            [](const char c) { return c == 'e' || c == 'E'; });
+        if(decpoint != str.end() && exponent != str.end())
+        {
+            assert(decpoint < exponent);
+        }
+
+        if(fmt.fmt == floating_format::scientific)
+        {
+            // total width
+            fmt.prec = static_cast<std::size_t>(std::distance(str.begin(), exponent));
+            if(has_sign)
+            {
+                fmt.prec -= 1;
+            }
+            if(decpoint != str.end())
+            {
+                fmt.prec -= 1;
+            }
+        }
+        else if(fmt.fmt == floating_format::hex)
+        {
+            fmt.prec = std::numeric_limits<floating_type>::max_digits10;
+        }
+        else
+        {
+            // width after decimal point
+            fmt.prec = static_cast<std::size_t>(std::distance(std::next(decpoint), exponent));
+        }
+
+        auto src = source_location(region(loc));
+        const auto res = TC::parse_float(str, src, is_hex);
+        if(res.is_ok())
+        {
+            val = res.as_ok();
+        }
+        else
+        {
+            return err(res.as_err());
+        }
+    }
+
+    // ----------------------------------------------------------------------
+    // parse suffix (extension)
+
+    if(spec.ext_num_suffix && loc.current() == '_')
+    {
+        const auto sfx_reg = syntax::num_suffix(spec).scan(loc);
+        if( ! sfx_reg.is_ok())
+        {
+            auto src = source_location(region(loc));
+            loc = first;
+            return err(make_error_info("toml::parse_floating: "
+                "invalid suffix: should be `_ non-digit-graph (graph | _graph)`",
+                std::move(src), "here"));
+        }
+        auto sfx = sfx_reg.as_string();
+        assert( ! sfx.empty() && sfx.front() == '_');
+        sfx.erase(sfx.begin()); // remove the first `_`
+
+        fmt.suffix = sfx;
+    }
+
+    return ok(basic_value<TC>(val, std::move(fmt), {}, std::move(reg)));
+}
+
+/* ============================================================================
+ *  ___       _       _   _
+ * |   \ __ _| |_ ___| |_(_)_ __  ___
+ * | |) / _` |  _/ -_)  _| | '  \/ -_)
+ * |___/\__,_|\__\___|\__|_|_|_|_\___|
+ */
+
+// all the offset_datetime, local_datetime, local_date parses date part.
+template<typename TC>
+result<std::tuple<local_date, local_date_format_info, region>, error_info>
+parse_local_date_only(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    local_date_format_info fmt;
+
+    // ----------------------------------------------------------------------
+    // check syntax
+    auto reg = syntax::local_date(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_local_date: "
+            "invalid date: date must be like: 1234-05-06, yyyy-mm-dd.",
+            syntax::local_date(spec), loc));
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+    const auto str = reg.as_string();
+
+    // 0123456789
+    // yyyy-mm-dd
+    const auto year_r  = from_string<int>(str.substr(0, 4));
+    const auto month_r = from_string<int>(str.substr(5, 2));
+    const auto day_r   = from_string<int>(str.substr(8, 2));
+
+    if(year_r.is_err())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_date: "
+            "failed to read year `" + str.substr(0, 4) + "`",
+            std::move(src), "here"));
+    }
+    if(month_r.is_err())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_date: "
+            "failed to read month `" + str.substr(5, 2) + "`",
+            std::move(src), "here"));
+    }
+    if(day_r.is_err())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_date: "
+            "failed to read day `" + str.substr(8, 2) + "`",
+            std::move(src), "here"));
+    }
+
+    const auto year  = year_r.unwrap();
+    const auto month = month_r.unwrap();
+    const auto day   = day_r.unwrap();
+
+    {
+        // We briefly check whether the input date is valid or not.
+        //     Actually, because of the historical reasons, there are several
+        // edge cases, such as 1582/10/5-1582/10/14 (only in several countries).
+        // But here, we do not care about it.
+        // It makes the code complicated and there is only low probability
+        // that such a specific date is needed in practice. If someone need to
+        // validate date accurately, that means that the one need a specialized
+        // library for their purpose in another layer.
+
+        const bool is_leap = (year % 4 == 0) && ((year % 100 != 0) || (year % 400 == 0));
+        const auto max_day = [month, is_leap]() {
+            if(month == 2)
+            {
+                return is_leap ? 29 : 28;
+            }
+            if(month == 4 || month == 6 || month == 9 || month == 11)
+            {
+                return 30;
+            }
+            return 31;
+        }();
+
+        if((month < 1 || 12 < month) || (day < 1 || max_day < day))
+        {
+            auto src = source_location(region(first));
+            return err(make_error_info("toml::parse_local_date: invalid date.",
+                std::move(src), "month must be 01-12, day must be any of "
+                "01-28,29,30,31 depending on the month/year."));
+        }
+    }
+
+    return ok(std::make_tuple(
+            local_date(year, static_cast<month_t>(month - 1), day),
+            std::move(fmt), std::move(reg)
+        ));
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_local_date(location& loc, const context<TC>& ctx)
+{
+    auto val_fmt_reg = parse_local_date_only(loc, ctx);
+    if(val_fmt_reg.is_err())
+    {
+        return err(val_fmt_reg.unwrap_err());
+    }
+
+    auto val = std::move(std::get<0>(val_fmt_reg.unwrap()));
+    auto fmt = std::move(std::get<1>(val_fmt_reg.unwrap()));
+    auto reg = std::move(std::get<2>(val_fmt_reg.unwrap()));
+
+    return ok(basic_value<TC>(std::move(val), std::move(fmt), {}, std::move(reg)));
+}
+
+// all the offset_datetime, local_datetime, local_time parses date part.
+template<typename TC>
+result<std::tuple<local_time, local_time_format_info, region>, error_info>
+parse_local_time_only(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    local_time_format_info fmt;
+
+    // ----------------------------------------------------------------------
+    // check syntax
+    auto reg = syntax::local_time(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        if(spec.v1_1_0_make_seconds_optional)
+        {
+            return err(make_syntax_error("toml::parse_local_time: "
+                "invalid time: time must be HH:MM(:SS.sss) (seconds are optional)",
+                syntax::local_time(spec), loc));
+        }
+        else
+        {
+            return err(make_syntax_error("toml::parse_local_time: "
+                "invalid time: time must be HH:MM:SS(.sss) (subseconds are optional)",
+                syntax::local_time(spec), loc));
+        }
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+    const auto str = reg.as_string();
+
+    // at least we have HH:MM.
+    // 01234
+    // HH:MM
+    const auto hour_r   = from_string<int>(str.substr(0, 2));
+    const auto minute_r = from_string<int>(str.substr(3, 2));
+
+    if(hour_r.is_err())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_time: "
+            "failed to read hour `" + str.substr(0, 2) + "`",
+            std::move(src), "here"));
+    }
+    if(minute_r.is_err())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_time: "
+            "failed to read minute `" + str.substr(3, 2) + "`",
+            std::move(src), "here"));
+    }
+
+    const auto hour   = hour_r.unwrap();
+    const auto minute = minute_r.unwrap();
+
+    if((hour < 0 || 24 <= hour) || (minute < 0 || 60 <= minute))
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_time: invalid time.",
+            std::move(src), "hour must be 00-23, minute must be 00-59."));
+    }
+
+    // -----------------------------------------------------------------------
+    // we have hour and minute.
+    // Since toml v1.1.0, second and subsecond part becomes optional.
+    // Check the version and return if second does not exist.
+
+    if(str.size() == 5 && spec.v1_1_0_make_seconds_optional)
+    {
+        fmt.has_seconds = false;
+        fmt.subsecond_precision = 0;
+        return ok(std::make_tuple(local_time(hour, minute, 0), std::move(fmt), std::move(reg)));
+    }
+    assert(str.at(5) == ':');
+
+    // we have at least `:SS` part. `.subseconds` are optional.
+
+    // 0         1
+    // 012345678901234
+    // HH:MM:SS.subsec
+    const auto sec_r = from_string<int>(str.substr(6, 2));
+    if(sec_r.is_err())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_time: "
+            "failed to read second `" + str.substr(6, 2) + "`",
+            std::move(src), "here"));
+    }
+    const auto sec = sec_r.unwrap();
+
+    if(sec < 0 || 60 < sec) // :60 is allowed
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_time: invalid time.",
+                    std::move(src), "second must be 00-60."));
+    }
+
+    if(str.size() == 8)
+    {
+        fmt.has_seconds = true;
+        fmt.subsecond_precision = 0;
+        return ok(std::make_tuple(local_time(hour, minute, sec), std::move(fmt), std::move(reg)));
+    }
+
+    assert(str.at(8) == '.');
+
+    auto secfrac = str.substr(9, str.size() - 9);
+
+    fmt.has_seconds = true;
+    fmt.subsecond_precision = secfrac.size();
+
+    while(secfrac.size() < 9)
+    {
+        secfrac += '0';
+    }
+    assert(9 <= secfrac.size());
+    const auto ms_r = from_string<int>(secfrac.substr(0, 3));
+    const auto us_r = from_string<int>(secfrac.substr(3, 3));
+    const auto ns_r = from_string<int>(secfrac.substr(6, 3));
+
+    if(ms_r.is_err())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_time: "
+            "failed to read milliseconds `" + secfrac.substr(0, 3) + "`",
+            std::move(src), "here"));
+    }
+    if(us_r.is_err())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_time: "
+            "failed to read microseconds`" + str.substr(3, 3) + "`",
+            std::move(src), "here"));
+    }
+    if(ns_r.is_err())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_local_time: "
+            "failed to read nanoseconds`" + str.substr(6, 3) + "`",
+            std::move(src), "here"));
+    }
+    const auto ms = ms_r.unwrap();
+    const auto us = us_r.unwrap();
+    const auto ns = ns_r.unwrap();
+
+    return ok(std::make_tuple(local_time(hour, minute, sec, ms, us, ns), std::move(fmt), std::move(reg)));
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_local_time(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+
+    auto val_fmt_reg = parse_local_time_only(loc, ctx);
+    if(val_fmt_reg.is_err())
+    {
+        return err(val_fmt_reg.unwrap_err());
+    }
+
+    auto val = std::move(std::get<0>(val_fmt_reg.unwrap()));
+    auto fmt = std::move(std::get<1>(val_fmt_reg.unwrap()));
+    auto reg = std::move(std::get<2>(val_fmt_reg.unwrap()));
+
+    return ok(basic_value<TC>(std::move(val), std::move(fmt), {}, std::move(reg)));
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_local_datetime(location& loc, const context<TC>& ctx)
+{
+    using char_type = location::char_type;
+
+    const auto first = loc;
+
+    local_datetime_format_info fmt;
+
+    // ----------------------------------------------------------------------
+
+    auto date_fmt_reg = parse_local_date_only(loc, ctx);
+    if(date_fmt_reg.is_err())
+    {
+        return err(date_fmt_reg.unwrap_err());
+    }
+
+    if(loc.current() == char_type('T'))
+    {
+        loc.advance();
+        fmt.delimiter = datetime_delimiter_kind::upper_T;
+    }
+    else if(loc.current() == char_type('t'))
+    {
+        loc.advance();
+        fmt.delimiter = datetime_delimiter_kind::lower_t;
+    }
+    else if(loc.current() == char_type(' '))
+    {
+        loc.advance();
+        fmt.delimiter = datetime_delimiter_kind::space;
+    }
+    else
+    {
+        auto src = source_location(region(loc));
+        return err(make_error_info("toml::parse_local_datetime: "
+            "expect date-time delimiter `T`, `t` or ` `(space).",
+            std::move(src), "here"));
+    }
+
+    auto time_fmt_reg = parse_local_time_only(loc, ctx);
+    if(time_fmt_reg.is_err())
+    {
+        return err(time_fmt_reg.unwrap_err());
+    }
+
+    fmt.has_seconds         = std::get<1>(time_fmt_reg.unwrap()).has_seconds;
+    fmt.subsecond_precision = std::get<1>(time_fmt_reg.unwrap()).subsecond_precision;
+
+    // ----------------------------------------------------------------------
+
+    region reg(first, loc);
+    local_datetime val(std::get<0>(date_fmt_reg.unwrap()),
+                       std::get<0>(time_fmt_reg.unwrap()));
+
+    return ok(basic_value<TC>(val, std::move(fmt), {}, std::move(reg)));
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_offset_datetime(location& loc, const context<TC>& ctx)
+{
+    using char_type = location::char_type;
+
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    offset_datetime_format_info fmt;
+
+    // ----------------------------------------------------------------------
+    // date part
+
+    auto date_fmt_reg = parse_local_date_only(loc, ctx);
+    if(date_fmt_reg.is_err())
+    {
+        return err(date_fmt_reg.unwrap_err());
+    }
+
+    // ----------------------------------------------------------------------
+    // delimiter
+
+    if(loc.current() == char_type('T'))
+    {
+        loc.advance();
+        fmt.delimiter = datetime_delimiter_kind::upper_T;
+    }
+    else if(loc.current() == char_type('t'))
+    {
+        loc.advance();
+        fmt.delimiter = datetime_delimiter_kind::lower_t;
+    }
+    else if(loc.current() == char_type(' '))
+    {
+        loc.advance();
+        fmt.delimiter = datetime_delimiter_kind::space;
+    }
+    else
+    {
+        auto src = source_location(region(loc));
+        return err(make_error_info("toml::parse_offset_datetime: "
+            "expect date-time delimiter `T` or ` `(space).", std::move(src), "here"
+        ));
+    }
+
+    // ----------------------------------------------------------------------
+    // time part
+
+    auto time_fmt_reg = parse_local_time_only(loc, ctx);
+    if(time_fmt_reg.is_err())
+    {
+        return err(time_fmt_reg.unwrap_err());
+    }
+
+    fmt.has_seconds         = std::get<1>(time_fmt_reg.unwrap()).has_seconds;
+    fmt.subsecond_precision = std::get<1>(time_fmt_reg.unwrap()).subsecond_precision;
+
+    // ----------------------------------------------------------------------
+    // offset part
+
+    const auto ofs_reg = syntax::time_offset(spec).scan(loc);
+    if( ! ofs_reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_offset_datetime: "
+            "invalid offset: offset must be like: Z, +01:00, or -10:00.",
+            syntax::time_offset(spec), loc));
+    }
+
+    const auto ofs_str = ofs_reg.as_string();
+
+    time_offset offset(0, 0);
+
+    assert(ofs_str.size() != 0);
+
+    if(ofs_str.at(0) == char_type('+') || ofs_str.at(0) == char_type('-'))
+    {
+        const auto hour_r   = from_string<int>(ofs_str.substr(1, 2));
+        const auto minute_r = from_string<int>(ofs_str.substr(4, 2));
+        if(hour_r.is_err())
+        {
+            auto src = source_location(region(loc));
+            return err(make_error_info("toml::parse_offset_datetime: "
+                "Failed to read offset hour part", std::move(src), "here"));
+        }
+        if(minute_r.is_err())
+        {
+            auto src = source_location(region(loc));
+            return err(make_error_info("toml::parse_offset_datetime: "
+                "Failed to read offset minute part", std::move(src), "here"));
+        }
+        const auto hour   = hour_r.unwrap();
+        const auto minute = minute_r.unwrap();
+
+        if(ofs_str.at(0) == '+')
+        {
+            offset = time_offset(hour, minute);
+        }
+        else
+        {
+            offset = time_offset(-hour, -minute);
+        }
+    }
+    else
+    {
+        assert(ofs_str.at(0) == char_type('Z') || ofs_str.at(0) == char_type('z'));
+    }
+
+    if (offset.hour   < -24 || 24 < offset.hour ||
+        offset.minute < -60 || 60 < offset.minute)
+    {
+        return err(make_error_info("toml::parse_offset_datetime: "
+            "too large offset: |hour| <= 24, |minute| <= 60",
+            source_location(region(first, loc)), "here"));
+    }
+
+
+    // ----------------------------------------------------------------------
+
+    region reg(first, loc);
+    offset_datetime val(local_datetime(std::get<0>(date_fmt_reg.unwrap()),
+                                       std::get<0>(time_fmt_reg.unwrap())),
+                                       offset);
+
+    return ok(basic_value<TC>(val, std::move(fmt), {}, std::move(reg)));
+}
+
+/* ============================================================================
+ *   ___ _       _
+ *  / __| |_ _ _(_)_ _  __ _
+ *  \__ \  _| '_| | ' \/ _` |
+ *  |___/\__|_| |_|_||_\__, |
+ *                     |___/
+ */
+
+template<typename TC>
+result<typename basic_value<TC>::string_type, error_info>
+parse_utf8_codepoint(const region& reg)
+{
+    using string_type = typename basic_value<TC>::string_type;
+    using char_type = typename string_type::value_type;
+
+    // assert(reg.as_lines().size() == 1); // XXX heavy check
+
+    const auto str = reg.as_string();
+    assert( ! str.empty());
+    assert(str.front() == 'u' || str.front() == 'U' || str.front() == 'x');
+
+    std::uint_least32_t codepoint;
+    std::istringstream iss(str.substr(1));
+    iss >> std::hex >> codepoint;
+
+    const auto to_char = [](const std::uint_least32_t i) noexcept -> char_type {
+        const auto uc = static_cast<unsigned char>(i & 0xFF);
+        return cxx::bit_cast<char_type>(uc);
+    };
+
+    string_type character;
+    if(codepoint < 0x80) // U+0000 ... U+0079 ; just an ASCII.
+    {
+        character += static_cast<char>(codepoint);
+    }
+    else if(codepoint < 0x800) //U+0080 ... U+07FF
+    {
+        // 110yyyyx 10xxxxxx; 0x3f == 0b0011'1111
+        character += to_char(0xC0|(codepoint >> 6  ));
+        character += to_char(0x80|(codepoint & 0x3F));
+    }
+    else if(codepoint < 0x10000) // U+0800...U+FFFF
+    {
+        if(0xD800 <= codepoint && codepoint <= 0xDFFF)
+        {
+            auto src = source_location(reg);
+            return err(make_error_info("toml::parse_utf8_codepoint: "
+                "[0xD800, 0xDFFF] is not a valid UTF-8",
+                std::move(src), "here"));
+        }
+        assert(codepoint < 0xD800 || 0xDFFF < codepoint);
+        // 1110yyyy 10yxxxxx 10xxxxxx
+        character += to_char(0xE0| (codepoint >> 12));
+        character += to_char(0x80|((codepoint >> 6 ) & 0x3F));
+        character += to_char(0x80|((codepoint      ) & 0x3F));
+    }
+    else if(codepoint < 0x110000) // U+010000 ... U+10FFFF
+    {
+        // 11110yyy 10yyxxxx 10xxxxxx 10xxxxxx
+        character += to_char(0xF0| (codepoint >> 18));
+        character += to_char(0x80|((codepoint >> 12) & 0x3F));
+        character += to_char(0x80|((codepoint >> 6 ) & 0x3F));
+        character += to_char(0x80|((codepoint      ) & 0x3F));
+    }
+    else // out of UTF-8 region
+    {
+        auto src = source_location(reg);
+        return err(make_error_info("toml::parse_utf8_codepoint: "
+            "input codepoint is too large.",
+            std::move(src), "must be in range [0x00, 0x10FFFF]"));
+    }
+    return ok(character);
+}
+
+template<typename TC>
+result<typename basic_value<TC>::string_type, error_info>
+parse_escape_sequence(location& loc, const context<TC>& ctx)
+{
+    using string_type = typename basic_value<TC>::string_type;
+    using char_type = typename string_type::value_type;
+
+    const auto& spec = ctx.toml_spec();
+
+    assert( ! loc.eof());
+    assert(loc.current() == '\\');
+    loc.advance(); // consume the first backslash
+
+    string_type retval;
+
+    if     (loc.current() == '\\') { retval += char_type('\\'); loc.advance(); }
+    else if(loc.current() == '"')  { retval += char_type('\"'); loc.advance(); }
+    else if(loc.current() == 'b')  { retval += char_type('\b'); loc.advance(); }
+    else if(loc.current() == 'f')  { retval += char_type('\f'); loc.advance(); }
+    else if(loc.current() == 'n')  { retval += char_type('\n'); loc.advance(); }
+    else if(loc.current() == 'r')  { retval += char_type('\r'); loc.advance(); }
+    else if(loc.current() == 't')  { retval += char_type('\t'); loc.advance(); }
+    else if(spec.v1_1_0_add_escape_sequence_e && loc.current() == 'e')
+    {
+        retval += char_type('\x1b');
+        loc.advance();
+    }
+    else if(spec.v1_1_0_add_escape_sequence_x && loc.current() == 'x')
+    {
+        auto scanner = sequence(character('x'), repeat_exact(2, syntax::hexdig(spec)));
+        const auto reg = scanner.scan(loc);
+        if( ! reg.is_ok())
+        {
+            auto src = source_location(region(loc));
+            return err(make_error_info("toml::parse_escape_sequence: "
+                   "invalid token found in UTF-8 codepoint \\xhh",
+                   std::move(src), "here"));
+        }
+        const auto utf8 = parse_utf8_codepoint<TC>(reg);
+        if(utf8.is_err())
+        {
+            return err(utf8.as_err());
+        }
+        retval += utf8.unwrap();
+    }
+    else if(loc.current() == 'u')
+    {
+        auto scanner = sequence(character('u'), repeat_exact(4, syntax::hexdig(spec)));
+        const auto reg = scanner.scan(loc);
+        if( ! reg.is_ok())
+        {
+            auto src = source_location(region(loc));
+            return err(make_error_info("toml::parse_escape_sequence: "
+                   "invalid token found in UTF-8 codepoint \\uhhhh",
+                   std::move(src), "here"));
+        }
+        const auto utf8 = parse_utf8_codepoint<TC>(reg);
+        if(utf8.is_err())
+        {
+            return err(utf8.as_err());
+        }
+        retval += utf8.unwrap();
+    }
+    else if(loc.current() == 'U')
+    {
+        auto scanner = sequence(character('U'), repeat_exact(8, syntax::hexdig(spec)));
+        const auto reg = scanner.scan(loc);
+        if( ! reg.is_ok())
+        {
+            auto src = source_location(region(loc));
+            return err(make_error_info("toml::parse_escape_sequence: "
+                   "invalid token found in UTF-8 codepoint \\Uhhhhhhhh",
+                   std::move(src), "here"));
+        }
+        const auto utf8 = parse_utf8_codepoint<TC>(reg);
+        if(utf8.is_err())
+        {
+            return err(utf8.as_err());
+        }
+        retval += utf8.unwrap();
+    }
+    else
+    {
+        auto src = source_location(region(loc));
+        std::string escape_seqs = "allowed escape seqs: \\\\, \\\", \\b, \\f, \\n, \\r, \\t";
+        if(spec.v1_1_0_add_escape_sequence_e)
+        {
+            escape_seqs += ", \\e";
+        }
+        if(spec.v1_1_0_add_escape_sequence_x)
+        {
+            escape_seqs += ", \\xhh";
+        }
+        escape_seqs += ", \\uhhhh, or \\Uhhhhhhhh";
+
+        return err(make_error_info("toml::parse_escape_sequence: "
+               "unknown escape sequence.", std::move(src), escape_seqs));
+    }
+    return ok(retval);
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_ml_basic_string(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    string_format_info fmt;
+    fmt.fmt = string_format::multiline_basic;
+
+    auto reg = syntax::ml_basic_string(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_ml_basic_string: "
+            "invalid string format",
+            syntax::ml_basic_string(spec), loc));
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+
+    auto str = reg.as_string();
+
+    // we already checked that it starts with """ and ends with """.
+    assert(str.substr(0, 3) == "\"\"\"");
+    str.erase(0, 3);
+
+    assert(str.size() >= 3);
+    assert(str.substr(str.size()-3, 3) == "\"\"\"");
+    str.erase(str.size()-3, 3);
+
+    // the first newline just after """ is trimmed
+    if(str.size() >= 1 && str.at(0) == '\n')
+    {
+        str.erase(0, 1);
+        fmt.start_with_newline = true;
+    }
+    else if(str.size() >= 2 && str.at(0) == '\r' && str.at(1) == '\n')
+    {
+        str.erase(0, 2);
+        fmt.start_with_newline = true;
+    }
+
+    using string_type = typename basic_value<TC>::string_type;
+    string_type val;
+    {
+        auto iter = str.cbegin();
+        while(iter != str.cend())
+        {
+            if(*iter == '\\') // remove whitespaces around escaped-newline
+            {
+                // we assume that the string is not too long to copy
+                auto loc2 = make_temporary_location(make_string(iter, str.cend()));
+                if(syntax::escaped_newline(spec).scan(loc2).is_ok())
+                {
+                    std::advance(iter, loc2.get_location()); // skip escaped newline and indent
+                    // now iter points non-WS char
+                    assert(iter == str.end() || (*iter != ' ' && *iter != '\t'));
+                }
+                else // normal escape seq.
+                {
+                    auto esc = parse_escape_sequence(loc2, ctx);
+
+                    // syntax does not check its value. the unicode codepoint may be
+                    // invalid, e.g. out-of-bound, [0xD800, 0xDFFF]
+                    if(esc.is_err())
+                    {
+                        return err(esc.unwrap_err());
+                    }
+
+                    val += esc.unwrap();
+                    std::advance(iter, loc2.get_location());
+                }
+            }
+            else // we already checked the syntax. we don't need to check it again.
+            {
+                val += static_cast<typename string_type::value_type>(*iter);
+                ++iter;
+            }
+        }
+    }
+
+    return ok(basic_value<TC>(
+            std::move(val), std::move(fmt), {}, std::move(reg)
+        ));
+}
+
+template<typename TC>
+result<std::pair<typename basic_value<TC>::string_type, region>, error_info>
+parse_basic_string_only(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    auto reg = syntax::basic_string(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_basic_string: "
+            "invalid string format",
+            syntax::basic_string(spec), loc));
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+
+    auto str = reg.as_string();
+
+    assert(str.back() == '\"');
+    str.pop_back();
+    assert(str.at(0) == '\"');
+    str.erase(0, 1);
+
+    using string_type = typename basic_value<TC>::string_type;
+    using char_type   = typename string_type::value_type;
+    string_type val;
+
+    {
+        auto iter = str.begin();
+        while(iter != str.end())
+        {
+            if(*iter == '\\')
+            {
+                auto loc2 = make_temporary_location(make_string(iter, str.end()));
+
+                auto esc = parse_escape_sequence(loc2, ctx);
+
+                // syntax does not check its value. the unicode codepoint may be
+                // invalid, e.g. out-of-bound, [0xD800, 0xDFFF]
+                if(esc.is_err())
+                {
+                    return err(esc.unwrap_err());
+                }
+
+                val += esc.unwrap();
+                std::advance(iter, loc2.get_location());
+            }
+            else
+            {
+                val += char_type(*iter); // we already checked the syntax.
+                ++iter;
+            }
+        }
+    }
+    return ok(std::make_pair(val, reg));
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_basic_string(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+
+    string_format_info fmt;
+    fmt.fmt = string_format::basic;
+
+    auto val_res = parse_basic_string_only(loc, ctx);
+    if(val_res.is_err())
+    {
+        return err(std::move(val_res.unwrap_err()));
+    }
+    auto val = std::move(val_res.unwrap().first );
+    auto reg = std::move(val_res.unwrap().second);
+
+    return ok(basic_value<TC>(std::move(val), std::move(fmt), {}, std::move(reg)));
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_ml_literal_string(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    string_format_info fmt;
+    fmt.fmt = string_format::multiline_literal;
+
+    auto reg = syntax::ml_literal_string(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_ml_literal_string: "
+            "invalid string format",
+            syntax::ml_literal_string(spec), loc));
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+
+    auto str = reg.as_string();
+
+    assert(str.substr(0, 3) == "'''");
+    assert(str.substr(str.size()-3, 3) == "'''");
+    str.erase(0, 3);
+    str.erase(str.size()-3, 3);
+
+    // the first newline just after """ is trimmed
+    if(str.size() >= 1 && str.at(0) == '\n')
+    {
+        str.erase(0, 1);
+        fmt.start_with_newline = true;
+    }
+    else if(str.size() >= 2 && str.at(0) == '\r' && str.at(1) == '\n')
+    {
+        str.erase(0, 2);
+        fmt.start_with_newline = true;
+    }
+
+    using string_type = typename basic_value<TC>::string_type;
+    string_type val(str.begin(), str.end());
+
+    return ok(basic_value<TC>(
+            std::move(val), std::move(fmt), {}, std::move(reg)
+        ));
+}
+
+template<typename TC>
+result<std::pair<typename basic_value<TC>::string_type, region>, error_info>
+parse_literal_string_only(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    auto reg = syntax::literal_string(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_literal_string: "
+            "invalid string format",
+            syntax::literal_string(spec), loc));
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+
+    auto str = reg.as_string();
+
+    assert(str.back() == '\'');
+    str.pop_back();
+    assert(str.at(0) == '\'');
+    str.erase(0, 1);
+
+    using string_type = typename basic_value<TC>::string_type;
+    string_type val(str.begin(), str.end());
+
+    return ok(std::make_pair(std::move(val), std::move(reg)));
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_literal_string(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+
+    string_format_info fmt;
+    fmt.fmt = string_format::literal;
+
+    auto val_res = parse_literal_string_only(loc, ctx);
+    if(val_res.is_err())
+    {
+        return err(std::move(val_res.unwrap_err()));
+    }
+    auto val = std::move(val_res.unwrap().first );
+    auto reg = std::move(val_res.unwrap().second);
+
+    return ok(basic_value<TC>(
+            std::move(val), std::move(fmt), {}, std::move(reg)
+        ));
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_string(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+
+    if( ! loc.eof() && loc.current() == '"')
+    {
+        if(literal("\"\"\"").scan(loc).is_ok())
+        {
+            loc = first;
+            return parse_ml_basic_string(loc, ctx);
+        }
+        else
+        {
+            loc = first;
+            return parse_basic_string(loc, ctx);
+        }
+    }
+    else if( ! loc.eof() && loc.current() == '\'')
+    {
+        if(literal("'''").scan(loc).is_ok())
+        {
+            loc = first;
+            return parse_ml_literal_string(loc, ctx);
+        }
+        else
+        {
+            loc = first;
+            return parse_literal_string(loc, ctx);
+        }
+    }
+    else
+    {
+        auto src = source_location(region(loc));
+        return err(make_error_info("toml::parse_string: "
+            "not a string", std::move(src), "here"));
+    }
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_null(location& loc, const context<TC>& ctx)
+{
+    const auto& spec = ctx.toml_spec();
+    if( ! spec.ext_null_value)
+    {
+        return err(make_error_info("toml::parse_null: "
+            "invalid spec: spec.ext_null_value must be true.",
+            source_location(region(loc)), "here"));
+    }
+
+    // ----------------------------------------------------------------------
+    // check syntax
+    auto reg = syntax::null_value(spec).scan(loc);
+    if( ! reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_null: "
+            "invalid null: null must be lowercase. ",
+            syntax::null_value(spec), loc));
+    }
+
+    // ----------------------------------------------------------------------
+    // it matches. gen value
+
+    // ----------------------------------------------------------------------
+    // no format info for boolean
+
+    return ok(basic_value<TC>(detail::none_t{}, std::move(reg)));
+}
+
+/* ============================================================================
+ *   _  __
+ *  | |/ /___ _  _
+ *  | ' </ -_) || |
+ *  |_|\_\___|\_, |
+ *            |__/
+ */
+
+// non-dotted key.
+template<typename TC>
+result<typename basic_value<TC>::key_type, error_info>
+parse_simple_key(location& loc, const context<TC>& ctx)
+{
+    using key_type = typename basic_value<TC>::key_type;
+    const auto& spec = ctx.toml_spec();
+
+    if(loc.current() == '\"')
+    {
+        auto str_res = parse_basic_string_only(loc, ctx);
+        if(str_res.is_ok())
+        {
+            return ok(std::move(str_res.unwrap().first));
+        }
+        else
+        {
+            return err(std::move(str_res.unwrap_err()));
+        }
+    }
+    else if(loc.current() == '\'')
+    {
+        auto str_res = parse_literal_string_only(loc, ctx);
+        if(str_res.is_ok())
+        {
+            return ok(std::move(str_res.unwrap().first));
+        }
+        else
+        {
+            return err(std::move(str_res.unwrap_err()));
+        }
+    }
+
+    // bare key.
+
+    if(const auto bare = syntax::unquoted_key(spec).scan(loc))
+    {
+        return ok(string_conv<key_type>(bare.as_string()));
+    }
+    else
+    {
+        std::string postfix;
+        if(spec.v1_1_0_allow_non_english_in_bare_keys)
+        {
+            postfix = "Hint: Not all Unicode characters are allowed as bare key.\n";
+        }
+        else
+        {
+            postfix = "Hint: non-ASCII scripts are allowed in toml v1.1.0, but not in v1.0.0.\n";
+        }
+        return err(make_syntax_error("toml::parse_simple_key: "
+            "invalid key: key must be \"quoted\", 'quoted-literal', or bare key.",
+            syntax::unquoted_key(spec), loc, postfix));
+    }
+}
+
+// dotted key become vector of keys
+template<typename TC>
+result<std::pair<std::vector<typename basic_value<TC>::key_type>, region>, error_info>
+parse_key(location& loc, const context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    using key_type = typename basic_value<TC>::key_type;
+    std::vector<key_type> keys;
+    while( ! loc.eof())
+    {
+        auto key = parse_simple_key(loc, ctx);
+        if( ! key.is_ok())
+        {
+            return err(key.unwrap_err());
+        }
+        keys.push_back(std::move(key.unwrap()));
+
+        auto reg = syntax::dot_sep(spec).scan(loc);
+        if( ! reg.is_ok())
+        {
+            break;
+        }
+    }
+    if(keys.empty())
+    {
+        auto src = source_location(region(first));
+        return err(make_error_info("toml::parse_key: expected a new key, "
+                    "but got nothing", std::move(src), "reached EOF"));
+    }
+
+    return ok(std::make_pair(std::move(keys), region(first, loc)));
+}
+
+// ============================================================================
+
+// forward-decl to implement parse_array and parse_table
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_value(location&, context<TC>& ctx);
+
+template<typename TC>
+result<std::pair<
+        std::pair<std::vector<typename basic_value<TC>::key_type>, region>,
+        basic_value<TC>
+    >, error_info>
+parse_key_value_pair(location& loc, context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    auto key_res = parse_key(loc, ctx);
+    if(key_res.is_err())
+    {
+        loc = first;
+        return err(key_res.unwrap_err());
+    }
+
+    if( ! syntax::keyval_sep(spec).scan(loc).is_ok())
+    {
+        auto e = make_syntax_error("toml::parse_key_value_pair: "
+            "invalid key value separator `=`", syntax::keyval_sep(spec), loc);
+        loc = first;
+        return err(std::move(e));
+    }
+
+    auto v_res = parse_value(loc, ctx);
+    if(v_res.is_err())
+    {
+        // loc = first;
+        return err(v_res.unwrap_err());
+    }
+    return ok(std::make_pair(std::move(key_res.unwrap()), std::move(v_res.unwrap())));
+}
+
+/* ============================================================================
+ *    __ _ _ _ _ _ __ _ _  _
+ *   / _` | '_| '_/ _` | || |
+ *   \__,_|_| |_| \__,_|\_, |
+ *                      |__/
+ */
+
+// array(and multiline inline table with `{` and `}`) has the following format.
+// `[`
+// (ws|newline|comment-line)? (value) (ws|newline|comment-line)? `,`
+// (ws|newline|comment-line)? (value) (ws|newline|comment-line)? `,`
+// ...
+// (ws|newline|comment-line)? (value) (ws|newline|comment-line)? (`,`)?
+// (ws|newline|comment-line)? `]`
+// it skips (ws|newline|comment-line) and returns the token.
+template<typename TC>
+struct multiline_spacer
+{
+    using comment_type = typename TC::comment_type;
+    bool         newline_found;
+    indent_char  indent_type;
+    std::int32_t indent;
+    comment_type comments;
+};
+template<typename T>
+std::ostream& operator<<(std::ostream& os, const multiline_spacer<T>& sp)
+{
+    os << "{newline="    << sp.newline_found << ", ";
+    os << "indent_type=" << sp.indent_type << ", ";
+    os << "indent="      << sp.indent << ", ";
+    os << "comments="    << sp.comments.size() << "}";
+    return os;
+}
+
+template<typename TC>
+cxx::optional<multiline_spacer<TC>>
+skip_multiline_spacer(location& loc, context<TC>& ctx, const bool newline_found = false)
+{
+    const auto& spec = ctx.toml_spec();
+
+    multiline_spacer<TC> spacer;
+    spacer.newline_found = newline_found;
+    spacer.indent_type   = indent_char::none;
+    spacer.indent        = 0;
+    spacer.comments.clear();
+
+    bool spacer_found = false;
+    while( ! loc.eof())
+    {
+        if(auto comm = sequence(syntax::comment(spec), syntax::newline(spec)).scan(loc))
+        {
+            spacer.newline_found = true;
+            auto comment = comm.as_string();
+            if( ! comment.empty() && comment.back() == '\n')
+            {
+                comment.pop_back();
+                if (!comment.empty() && comment.back() == '\r')
+                {
+                    comment.pop_back();
+                }
+            }
+
+            spacer.comments.push_back(std::move(comment));
+            spacer.indent_type = indent_char::none;
+            spacer.indent = 0;
+            spacer_found = true;
+        }
+        else if(auto nl = syntax::newline(spec).scan(loc))
+        {
+            spacer.newline_found = true;
+            spacer.comments.clear();
+            spacer.indent_type = indent_char::none;
+            spacer.indent = 0;
+            spacer_found = true;
+        }
+        else if(auto sp = repeat_at_least(1, character(cxx::bit_cast<location::char_type>(' '))).scan(loc))
+        {
+            spacer.indent_type = indent_char::space;
+            spacer.indent      = static_cast<std::int32_t>(sp.length());
+            spacer_found = true;
+        }
+        else if(auto tabs = repeat_at_least(1, character(cxx::bit_cast<location::char_type>('\t'))).scan(loc))
+        {
+            spacer.indent_type = indent_char::tab;
+            spacer.indent      = static_cast<std::int32_t>(tabs.length());
+            spacer_found = true;
+        }
+        else
+        {
+            break; // done
+        }
+    }
+    if( ! spacer_found)
+    {
+        return cxx::make_nullopt();
+    }
+    return spacer;
+}
+
+// not an [[array.of.tables]]. It parses ["this", "type"]
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_array(location& loc, context<TC>& ctx)
+{
+    const auto num_errors = ctx.errors().size();
+
+    const auto first = loc;
+
+    if(loc.eof() || loc.current() != '[')
+    {
+        auto src = source_location(region(loc));
+        return err(make_error_info("toml::parse_array: "
+                "The next token is not an array", std::move(src), "here"));
+    }
+    loc.advance();
+
+    typename basic_value<TC>::array_type val;
+
+    array_format_info fmt;
+    fmt.fmt = array_format::oneline;
+    fmt.indent_type = indent_char::none;
+
+    auto spacer = skip_multiline_spacer(loc, ctx);
+    if(spacer.has_value() && spacer.value().newline_found)
+    {
+        fmt.fmt = array_format::multiline;
+    }
+
+    bool comma_found = true;
+    while( ! loc.eof())
+    {
+        if(loc.current() == location::char_type(']'))
+        {
+            if(spacer.has_value() && spacer.value().newline_found &&
+                spacer.value().indent_type != indent_char::none)
+            {
+                fmt.indent_type    = spacer.value().indent_type;
+                fmt.closing_indent = spacer.value().indent;
+            }
+            break;
+        }
+
+        if( ! comma_found)
+        {
+            auto src = source_location(region(loc));
+            return err(make_error_info("toml::parse_array: "
+                    "expected value-separator `,` or closing `]`",
+                    std::move(src), "here"));
+        }
+
+        if(spacer.has_value() && spacer.value().newline_found &&
+            spacer.value().indent_type != indent_char::none)
+        {
+            fmt.indent_type = spacer.value().indent_type;
+            fmt.body_indent = spacer.value().indent;
+        }
+
+        if(auto elem_res = parse_value(loc, ctx))
+        {
+            auto elem = std::move(elem_res.unwrap());
+
+            if(spacer.has_value()) // copy previous comments to value
+            {
+                elem.comments() = std::move(spacer.value().comments);
+            }
+
+            // parse spaces between a value and a comma
+            // array = [
+            //     42    , # the answer
+            //       ^^^^
+            //     3.14 # pi
+            //   , 2.71 ^^^^
+            // ^^
+            spacer = skip_multiline_spacer(loc, ctx);
+            if(spacer.has_value())
+            {
+                for(std::size_t i=0; i<spacer.value().comments.size(); ++i)
+                {
+                    elem.comments().push_back(std::move(spacer.value().comments.at(i)));
+                }
+                if(spacer.value().newline_found)
+                {
+                    fmt.fmt = array_format::multiline;
+                }
+            }
+
+            comma_found = character(',').scan(loc).is_ok();
+
+            // parse comment after a comma
+            // array = [
+            //     42    , # the answer
+            //             ^^^^^^^^^^^^
+            //     3.14 # pi
+            //          ^^^^
+            // ]
+            auto com_res = parse_comment_line(loc, ctx);
+            if(com_res.is_err())
+            {
+                ctx.report_error(com_res.unwrap_err());
+            }
+
+            const bool comment_found = com_res.is_ok() && com_res.unwrap().has_value();
+            if(comment_found)
+            {
+                fmt.fmt = array_format::multiline;
+                elem.comments().push_back(com_res.unwrap().value());
+            }
+            if(comma_found)
+            {
+                spacer = skip_multiline_spacer(loc, ctx, comment_found);
+                if(spacer.has_value() && spacer.value().newline_found)
+                {
+                    fmt.fmt = array_format::multiline;
+                }
+            }
+            val.push_back(std::move(elem));
+        }
+        else
+        {
+            // if err, push error to ctx and try recovery.
+            ctx.report_error(std::move(elem_res.unwrap_err()));
+
+            // if it looks like some value, then skip the value.
+            // otherwise, it may be a new key-value pair or a new table and
+            // the error is "missing closing ]". stop parsing.
+
+            const auto before_skip = loc.get_location();
+            skip_value(loc, ctx);
+            if(before_skip == loc.get_location()) // cannot skip! break...
+            {
+                break;
+            }
+        }
+    }
+
+    if(loc.current() != ']')
+    {
+        auto src = source_location(region(loc));
+        return err(make_error_info("toml::parse_array: missing closing bracket `]`",
+            std::move(src), "expected `]`, reached EOF"));
+    }
+    else
+    {
+        loc.advance();
+    }
+    // any error reported from this function
+    if(num_errors != ctx.errors().size())
+    {
+        assert(ctx.has_error()); // already reported
+        return err(ctx.errors().back());
+    }
+
+    return ok(basic_value<TC>(
+            std::move(val), std::move(fmt), {}, region(first, loc)
+        ));
+}
+
+/* ============================================================================
+ *   _      _ _            _        _    _
+ *  (_)_ _ | (_)_ _  ___  | |_ __ _| |__| |___
+ *  | | ' \| | | ' \/ -_) |  _/ _` | '_ \ / -_)
+ *  |_|_||_|_|_|_||_\___|  \__\__,_|_.__/_\___|
+ */
+
+// ----------------------------------------------------------------------------
+// insert_value is the most complicated part of the toml spec.
+//
+// To parse a toml file correctly, we sometimes need to check an exising value
+// is appendable or not.
+//
+// For example while parsing an inline array of tables,
+//
+// ```toml
+// aot = [
+//   {a = "foo"},
+//   {a = "bar", b = "baz"},
+// ]
+// ```
+//
+// this `aot` is appendable until parser reaches to `]`. After that, it becomes
+// non-appendable.
+//
+// On the other hand, a normal array of tables, such as
+//
+// ```toml
+// [[aot]]
+// a = "foo"
+//
+// [[aot]]
+// a = "bar"
+// b = "baz"
+// ```
+// This `[[aot]]` is appendable until the parser reaches to the EOF.
+//
+//
+// It becomes a bit more difficult in case of dotted keys.
+// In TOML, it is allowed to append a key-value pair to a table that is
+// *implicitly* defined by a subtable definitino.
+//
+// ```toml
+// [x.y.z]
+// w = 123
+//
+// [x]
+// a = "foo" # OK. x is defined implicitly by `[x.y.z]`.
+// ```
+//
+// But if the table is defined by a dotted keys, it is not appendable.
+//
+// ```toml
+// [x]
+// y.z.w = 123
+//
+// [x.y]
+// # ERROR. x.y is already defined by a dotted table in the previous table.
+// ```
+//
+// Also, reopening a table using dotted keys is invalid.
+//
+// ```toml
+// [x.y.z]
+// w = 123
+//
+// [x]
+// y.z.v = 42 # ERROR. [x.y.z] is already defined.
+// ```
+//
+//
+// ```toml
+// [a]
+// b.c = "foo"
+// b.d = "bar"
+// ```
+//
+//
+// ```toml
+// a.b = "foo"
+// [a]
+// c = "bar" # ERROR
+// ```
+//
+// In summary,
+// - a table must be defined only once.
+// - assignment to an exising table is possible only when:
+//   - defining a subtable [x.y] to an existing table [x].
+//   - defining supertable [x] explicitly after [x.y].
+//   - adding dotted keys in the same table.
+
+enum class inserting_value_kind : std::uint8_t
+{
+    std_table,   // insert [standard.table]
+    array_table, // insert [[array.of.tables]]
+    dotted_keys  // insert a.b.c = "this"
+};
+
+template<typename TC>
+result<basic_value<TC>*, error_info>
+insert_value(const inserting_value_kind kind,
+    typename basic_value<TC>::table_type* current_table_ptr,
+    const std::vector<typename basic_value<TC>::key_type>& keys, region key_reg,
+    basic_value<TC> val)
+{
+    using value_type = basic_value<TC>;
+    using array_type = typename basic_value<TC>::array_type;
+    using table_type = typename basic_value<TC>::table_type;
+
+    auto key_loc = source_location(key_reg);
+
+    assert( ! keys.empty());
+
+    // dotted key can insert to dotted key tables defined at the same level.
+    // dotted key can NOT reopen a table even if it is implcitly-defined one.
+    //
+    // [x.y.z] # define x and x.y implicitly.
+    // a = 42
+    //
+    // [x] # reopening implcitly defined table
+    // r.s.t = 3.14 # VALID r and r.s are new tables.
+    // r.s.u = 2.71 # VALID r and r.s are dotted-key tables. valid.
+    //
+    // y.z.b = "foo" # INVALID x.y.z are multiline table, not a dotted key.
+    // y.c   = "bar" # INVALID x.y is implicit multiline table, not a dotted key.
+
+    // a table cannot reopen dotted-key tables.
+    //
+    // [t1]
+    // t2.t3.v = 0
+    // [t1.t2] # INVALID t1.t2 is defined as a dotted-key table.
+
+    for(std::size_t i=0; i<keys.size(); ++i)
+    {
+        const auto& key = keys.at(i);
+        table_type& current_table = *current_table_ptr;
+
+        if(i+1 < keys.size()) // there are more keys. go down recursively...
+        {
+            const auto found = current_table.find(key);
+            if(found == current_table.end()) // not found. add new table
+            {
+                table_format_info fmt;
+                fmt.indent_type = indent_char::none;
+                if(kind == inserting_value_kind::dotted_keys)
+                {
+                    fmt.fmt = table_format::dotted;
+                }
+                else // table / array of tables
+                {
+                    fmt.fmt = table_format::implicit;
+                }
+                current_table.emplace(key, value_type(
+                    table_type{}, fmt, std::vector<std::string>{}, key_reg));
+
+                assert(current_table.at(key).is_table());
+                current_table_ptr = std::addressof(current_table.at(key).as_table());
+            }
+            else if (found->second.is_table())
+            {
+                const auto fmt = found->second.as_table_fmt().fmt;
+                if(fmt == table_format::oneline || fmt == table_format::multiline_oneline)
+                {
+                    // foo = {bar = "baz"} or foo = { \n bar = "baz" \n }
+                    return err(make_error_info("toml::insert_value: "
+                        "failed to insert a value: inline table is immutable",
+                        key_loc, "inserting this",
+                        found->second.location(), "to this table"));
+                }
+                // dotted key cannot reopen a table.
+                if(kind ==inserting_value_kind::dotted_keys && fmt != table_format::dotted)
+                {
+                    return err(make_error_info("toml::insert_value: "
+                        "reopening a table using dotted keys",
+                        key_loc, "dotted key cannot reopen a table",
+                        found->second.location(), "this table is already closed"));
+                }
+                assert(found->second.is_table());
+                current_table_ptr = std::addressof(found->second.as_table());
+            }
+            else if(found->second.is_array_of_tables())
+            {
+                // aot = [{this = "type", of = "aot"}] # cannot be reopened
+                if(found->second.as_array_fmt().fmt != array_format::array_of_tables)
+                {
+                    return err(make_error_info("toml::insert_value:"
+                        "inline array of tables are immutable",
+                        key_loc, "inserting this",
+                        found->second.location(), "inline array of tables"));
+                }
+                // appending to [[aot]]
+
+                if(kind == inserting_value_kind::dotted_keys)
+                {
+                    // [[array.of.tables]]
+                    // [array.of]          # reopening supertable is okay
+                    // tables.x = "foo"    # appending `x` to the first table
+                    return err(make_error_info("toml::insert_value:"
+                        "dotted key cannot reopen an array-of-tables",
+                        key_loc, "inserting this",
+                        found->second.location(), "to this array-of-tables."));
+                }
+
+                // insert_value_by_dotkeys::std_table
+                // [[array.of.tables]]
+                // [array.of.tables.subtable] # appending to the last aot
+                //
+                // insert_value_by_dotkeys::array_table
+                // [[array.of.tables]]
+                // [[array.of.tables.subtable]] # appending to the last aot
+                auto& current_array_table = found->second.as_array().back();
+
+                assert(current_array_table.is_table());
+                current_table_ptr = std::addressof(current_array_table.as_table());
+            }
+            else
+            {
+                return err(make_error_info("toml::insert_value: "
+                    "failed to insert a value, value already exists",
+                    key_loc, "while inserting this",
+                    found->second.location(), "non-table value already exists"));
+            }
+        }
+        else // this is the last key. insert a new value.
+        {
+            switch(kind)
+            {
+                case inserting_value_kind::dotted_keys:
+                {
+                    if(current_table.find(key) != current_table.end())
+                    {
+                        return err(make_error_info("toml::insert_value: "
+                            "failed to insert a value, value already exists",
+                            key_loc, "inserting this",
+                            current_table.at(key).location(), "but value already exists"));
+                    }
+                    current_table.emplace(key, std::move(val));
+                    return ok(std::addressof(current_table.at(key)));
+                }
+                case inserting_value_kind::std_table:
+                {
+                    // defining a new table or reopening supertable
+                    auto found = current_table.find(key);
+                    if(found == current_table.end()) // define a new aot
+                    {
+                        current_table.emplace(key, std::move(val));
+                        return ok(std::addressof(current_table.at(key)));
+                    }
+                    else // the table is already defined, reopen it
+                    {
+                        // assigning a [std.table]. it must be an implicit table.
+                        auto& target = found->second;
+                        if( ! target.is_table() || // could be an array-of-tables
+                            target.as_table_fmt().fmt != table_format::implicit)
+                        {
+                            return err(make_error_info("toml::insert_value: "
+                                "failed to insert a table, table already defined",
+                                key_loc, "inserting this",
+                                target.location(), "this table is explicitly defined"));
+                        }
+
+                        // merge table
+                        for(const auto& kv : val.as_table())
+                        {
+                            if(target.contains(kv.first))
+                            {
+                                // [x.y.z]
+                                // w = "foo"
+                                // [x]
+                                // y = "bar"
+                                return err(make_error_info("toml::insert_value: "
+                                    "failed to insert a table, table keys conflict to each other",
+                                    key_loc, "inserting this table",
+                                    kv.second.location(), "having this value",
+                                    target.at(kv.first).location(), "already defined here"));
+                            }
+                            else
+                            {
+                                target[kv.first] = kv.second;
+                            }
+                        }
+                        // change implicit -> explicit
+                        target.as_table_fmt().fmt = table_format::multiline;
+                        // change definition region
+                        change_region_of_value(target, val);
+
+                        return ok(std::addressof(current_table.at(key)));
+                    }
+                }
+                case inserting_value_kind::array_table:
+                {
+                    auto found = current_table.find(key);
+                    if(found == current_table.end()) // define a new aot
+                    {
+                        array_format_info fmt;
+                        fmt.fmt = array_format::array_of_tables;
+                        fmt.indent_type = indent_char::none;
+
+                        current_table.emplace(key, value_type(
+                                array_type{ std::move(val) }, std::move(fmt),
+                                std::vector<std::string>{}, std::move(key_reg)
+                            ));
+
+                        assert( ! current_table.at(key).as_array().empty());
+                        return ok(std::addressof(current_table.at(key).as_array().back()));
+                    }
+                    else // the array is already defined, append to it
+                    {
+                        if( ! found->second.is_array_of_tables())
+                        {
+                            return err(make_error_info("toml::insert_value: "
+                                "failed to insert an array of tables, value already exists",
+                                key_loc, "while inserting this",
+                                found->second.location(), "non-table value already exists"));
+                        }
+                        if(found->second.as_array_fmt().fmt != array_format::array_of_tables)
+                        {
+                            return err(make_error_info("toml::insert_value: "
+                                "failed to insert a table, inline array of tables is immutable",
+                                key_loc, "while inserting this",
+                                found->second.location(), "this is inline array-of-tables"));
+                        }
+                        found->second.as_array().push_back(std::move(val));
+                        assert( ! current_table.at(key).as_array().empty());
+                        return ok(std::addressof(current_table.at(key).as_array().back()));
+                    }
+                }
+                default: {assert(false);}
+            }
+        }
+    }
+    return err(make_error_info("toml::insert_key: no keys found",
+                std::move(key_loc), "here"));
+}
+
+// ----------------------------------------------------------------------------
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_inline_table(location& loc, context<TC>& ctx)
+{
+    using table_type = typename basic_value<TC>::table_type;
+
+    const auto num_errors = ctx.errors().size();
+
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    if(loc.eof() || loc.current() != '{')
+    {
+        auto src = source_location(region(loc));
+        return err(make_error_info("toml::parse_inline_table: "
+            "The next token is not an inline table", std::move(src), "here"));
+    }
+    loc.advance();
+
+    table_type table;
+    table_format_info fmt;
+    fmt.fmt = table_format::oneline;
+    fmt.indent_type = indent_char::none;
+
+    cxx::optional<multiline_spacer<TC>> spacer(cxx::make_nullopt());
+
+    if(spec.v1_1_0_allow_newlines_in_inline_tables)
+    {
+        spacer = skip_multiline_spacer(loc, ctx);
+        if(spacer.has_value() && spacer.value().newline_found)
+        {
+            fmt.fmt = table_format::multiline_oneline;
+        }
+    }
+    else
+    {
+        skip_whitespace(loc, ctx);
+    }
+
+    bool still_empty = true;
+    bool comma_found = false;
+    while( ! loc.eof())
+    {
+        // closing!
+        if(loc.current() == '}')
+        {
+            if(comma_found && ! spec.v1_1_0_allow_trailing_comma_in_inline_tables)
+            {
+                auto src = source_location(region(loc));
+                return err(make_error_info("toml::parse_inline_table: trailing "
+                    "comma is not allowed in TOML-v1.0.0)", std::move(src), "here"));
+            }
+
+            if(spec.v1_1_0_allow_newlines_in_inline_tables)
+            {
+                if(spacer.has_value() && spacer.value().newline_found &&
+                    spacer.value().indent_type != indent_char::none)
+                {
+                    fmt.indent_type    = spacer.value().indent_type;
+                    fmt.closing_indent = spacer.value().indent;
+                }
+            }
+            break;
+        }
+
+        // if we already found a value and didn't found `,` nor `}`, error.
+        if( ! comma_found && ! still_empty)
+        {
+            auto src = source_location(region(loc));
+            return err(make_error_info("toml::parse_inline_table: "
+                    "expected value-separator `,` or closing `}`",
+                    std::move(src), "here"));
+        }
+
+        // parse indent.
+        if(spacer.has_value() && spacer.value().newline_found &&
+            spacer.value().indent_type != indent_char::none)
+        {
+            fmt.indent_type = spacer.value().indent_type;
+            fmt.body_indent = spacer.value().indent;
+        }
+
+        still_empty = false; // parsing a value...
+        if(auto kv_res = parse_key_value_pair<TC>(loc, ctx))
+        {
+            auto keys    = std::move(kv_res.unwrap().first.first);
+            auto key_reg = std::move(kv_res.unwrap().first.second);
+            auto val     = std::move(kv_res.unwrap().second);
+
+            auto ins_res = insert_value(inserting_value_kind::dotted_keys,
+                std::addressof(table), keys, std::move(key_reg), std::move(val));
+            if(ins_res.is_err())
+            {
+                ctx.report_error(std::move(ins_res.unwrap_err()));
+                // we need to skip until the next value (or end of the table)
+                // because we don't have valid kv pair.
+                while( ! loc.eof())
+                {
+                    const auto c = loc.current();
+                    if(c == ',' || c == '\n' || c == '}')
+                    {
+                        comma_found = (c == ',');
+                        break;
+                    }
+                    loc.advance();
+                }
+                continue;
+            }
+
+            // if comment line follows immediately(without newline) after `,`, then
+            // the comment is for the elem. we need to check if comment follows `,`.
+            //
+            // (key) = (val) (ws|newline|comment-line)? `,` (ws)? (comment)?
+
+            if(spec.v1_1_0_allow_newlines_in_inline_tables)
+            {
+                if(spacer.has_value()) // copy previous comments to value
+                {
+                    for(std::size_t i=0; i<spacer.value().comments.size(); ++i)
+                    {
+                        ins_res.unwrap()->comments().push_back(spacer.value().comments.at(i));
+                    }
+                }
+                spacer = skip_multiline_spacer(loc, ctx);
+                if(spacer.has_value())
+                {
+                    for(std::size_t i=0; i<spacer.value().comments.size(); ++i)
+                    {
+                        ins_res.unwrap()->comments().push_back(spacer.value().comments.at(i));
+                    }
+                    if(spacer.value().newline_found)
+                    {
+                        fmt.fmt = table_format::multiline_oneline;
+                        if(spacer.value().indent_type != indent_char::none)
+                        {
+                            fmt.indent_type = spacer.value().indent_type;
+                            fmt.body_indent = spacer.value().indent;
+                        }
+                    }
+                }
+            }
+            else
+            {
+                skip_whitespace(loc, ctx);
+            }
+
+            comma_found = character(',').scan(loc).is_ok();
+
+            if(spec.v1_1_0_allow_newlines_in_inline_tables)
+            {
+                auto com_res = parse_comment_line(loc, ctx);
+                if(com_res.is_err())
+                {
+                    ctx.report_error(com_res.unwrap_err());
+                }
+                const bool comment_found = com_res.is_ok() && com_res.unwrap().has_value();
+                if(comment_found)
+                {
+                    fmt.fmt = table_format::multiline_oneline;
+                    ins_res.unwrap()->comments().push_back(com_res.unwrap().value());
+                }
+                if(comma_found)
+                {
+                    spacer = skip_multiline_spacer(loc, ctx, comment_found);
+                    if(spacer.has_value() && spacer.value().newline_found)
+                    {
+                        fmt.fmt = table_format::multiline_oneline;
+                    }
+                }
+            }
+            else
+            {
+                skip_whitespace(loc, ctx);
+            }
+        }
+        else
+        {
+            ctx.report_error(std::move(kv_res.unwrap_err()));
+            while( ! loc.eof())
+            {
+                if(loc.current() == '}')
+                {
+                    break;
+                }
+                if( ! spec.v1_1_0_allow_newlines_in_inline_tables && loc.current() == '\n')
+                {
+                    break;
+                }
+                loc.advance();
+            }
+            break;
+        }
+    }
+
+    if(loc.current() != '}')
+    {
+        auto src = source_location(region(loc));
+        return err(make_error_info("toml::parse_inline_table: "
+            "missing closing bracket `}`",
+            std::move(src), "expected `}`, reached line end"));
+    }
+    else
+    {
+        loc.advance(); // skip }
+    }
+
+    // any error reported from this function
+    if(num_errors < ctx.errors().size())
+    {
+        assert(ctx.has_error()); // already reported
+        return err(ctx.pop_last_error());
+    }
+
+    basic_value<TC> retval(
+        std::move(table), std::move(fmt), {}, region(first, loc));
+
+    return ok(std::move(retval));
+}
+
+/* ============================================================================
+ *            _
+ *  __ ____ _| |_  _ ___
+ *  \ V / _` | | || / -_)
+ *   \_/\__,_|_|\_,_\___|
+ */
+
+template<typename TC>
+result<value_t, error_info>
+guess_number_type(const location& first, const context<TC>& ctx)
+{
+    const auto& spec = ctx.toml_spec();
+    location loc = first;
+
+    if(syntax::offset_datetime(spec).scan(loc).is_ok())
+    {
+        return ok(value_t::offset_datetime);
+    }
+    loc = first;
+
+    if(syntax::local_datetime(spec).scan(loc).is_ok())
+    {
+        const auto curr = loc.current();
+        // if offset_datetime contains bad offset, it syntax::offset_datetime
+        // fails to scan it.
+        if(curr == '+' || curr == '-')
+        {
+            return err(make_syntax_error("bad offset: must be [+-]HH:MM or Z",
+                syntax::time_offset(spec), loc, std::string(
+                "Hint: valid  : +09:00, -05:30\n"
+                "Hint: invalid: +9:00,  -5:30\n")));
+        }
+        return ok(value_t::local_datetime);
+    }
+    loc = first;
+
+    if(syntax::local_date(spec).scan(loc).is_ok())
+    {
+        // bad time may appear after this.
+
+        if( ! loc.eof())
+        {
+            const auto c = loc.current();
+            if(c == 'T' || c == 't')
+            {
+                loc.advance();
+
+                return err(make_syntax_error("bad time: must be HH:MM:SS.subsec",
+                    syntax::local_time(spec), loc, std::string(
+                    "Hint: valid  : 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999\n"
+                    "Hint: invalid: 1979-05-27T7:32:00, 1979-05-27 17:32\n")));
+            }
+            if(c == ' ')
+            {
+                // A space is allowed as a delimiter between local time.
+                // But there is a case where bad time follows a space.
+                // - invalid: 2019-06-16 7:00:00
+                // - valid  : 2019-06-16 07:00:00
+                loc.advance();
+                if( ! loc.eof() && ('0' <= loc.current() && loc.current() <= '9'))
+                {
+                    return err(make_syntax_error("bad time: must be HH:MM:SS.subsec",
+                        syntax::local_time(spec), loc, std::string(
+                        "Hint: valid  : 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999\n"
+                        "Hint: invalid: 1979-05-27T7:32:00, 1979-05-27 17:32\n")));
+                }
+            }
+            if('0' <= c && c <= '9')
+            {
+                return err(make_syntax_error("bad datetime: missing T or space",
+                    character_either{'T', 't', ' '}, loc, std::string(
+                    "Hint: valid  : 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999\n"
+                    "Hint: invalid: 1979-05-27T7:32:00, 1979-05-27 17:32\n")));
+            }
+        }
+        return ok(value_t::local_date);
+    }
+    loc = first;
+
+    if(syntax::local_time(spec).scan(loc).is_ok())
+    {
+        return ok(value_t::local_time);
+    }
+    loc = first;
+
+    if(syntax::floating(spec).scan(loc).is_ok())
+    {
+        if( ! loc.eof() && loc.current() == '_')
+        {
+            if(spec.ext_num_suffix && syntax::num_suffix(spec).scan(loc).is_ok())
+            {
+                return ok(value_t::floating);
+            }
+            auto src = source_location(region(loc));
+            return err(make_error_info(
+                "bad float: `_` must be surrounded by digits",
+                std::move(src), "invalid underscore",
+                "Hint: valid  : +1.0, -2e-2, 3.141_592_653_589, inf, nan\n"
+                "Hint: invalid: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0\n"));
+        }
+        return ok(value_t::floating);
+    }
+    loc = first;
+
+    if(spec.ext_hex_float)
+    {
+        if(syntax::hex_floating(spec).scan(loc).is_ok())
+        {
+            if( ! loc.eof() && loc.current() == '_')
+            {
+                if(spec.ext_num_suffix && syntax::num_suffix(spec).scan(loc).is_ok())
+                {
+                    return ok(value_t::floating);
+                }
+                auto src = source_location(region(loc));
+                return err(make_error_info(
+                    "bad float: `_` must be surrounded by digits",
+                    std::move(src), "invalid underscore",
+                    "Hint: valid  : +1.0, -2e-2, 3.141_592_653_589, inf, nan\n"
+                    "Hint: invalid: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0\n"));
+            }
+            return ok(value_t::floating);
+        }
+        loc = first;
+    }
+
+    if(auto int_reg = syntax::integer(spec).scan(loc))
+    {
+        if( ! loc.eof())
+        {
+            const auto c = loc.current();
+            if(c == '_')
+            {
+                if(spec.ext_num_suffix && syntax::num_suffix(spec).scan(loc).is_ok())
+                {
+                    return ok(value_t::integer);
+                }
+
+                if(int_reg.length() <= 2 && (int_reg.as_string() == "0" ||
+                    int_reg.as_string() == "-0" || int_reg.as_string() == "+0"))
+                {
+                    auto src = source_location(region(loc));
+                    return err(make_error_info(
+                        "bad integer: leading zero is not allowed in decimal int",
+                        std::move(src), "leading zero",
+                        "Hint: valid  : -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755\n"
+                        "Hint: invalid: _42, 1__000, 0123\n"));
+                }
+                else
+                {
+                    auto src = source_location(region(loc));
+                    return err(make_error_info(
+                        "bad integer: `_` must be surrounded by digits",
+                        std::move(src), "invalid underscore",
+                        "Hint: valid  : -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755\n"
+                        "Hint: invalid: _42, 1__000, 0123\n"));
+                }
+            }
+            if('0' <= c && c <= '9')
+            {
+                if(loc.current() == '0')
+                {
+                    loc.retrace();
+                    return err(make_error_info(
+                        "bad integer: leading zero",
+                        source_location(region(loc)), "leading zero is not allowed",
+                        std::string("Hint: valid  : -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755\n"
+                                    "Hint: invalid: _42, 1__000, 0123\n")
+                        ));
+                }
+                else // invalid digits, especially in oct/bin ints.
+                {
+                    return err(make_error_info(
+                        "bad integer: invalid digit after an integer",
+                        source_location(region(loc)), "this digit is not allowed",
+                        std::string("Hint: valid  : -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755\n"
+                                    "Hint: invalid: _42, 1__000, 0123\n")
+                        ));
+                }
+            }
+            if(c == ':' || c == '-')
+            {
+                auto src = source_location(region(loc));
+                return err(make_error_info("bad datetime: invalid format",
+                    std::move(src), "here",
+                    std::string("Hint: valid  : 1979-05-27T07:32:00-07:00, 1979-05-27 07:32:00.999999Z\n"
+                                "Hint: invalid: 1979-05-27T7:32:00-7:00, 1979-05-27 7:32-00:30")
+                    ));
+            }
+            if(c == '.' || c == 'e' || c == 'E')
+            {
+                auto src = source_location(region(loc));
+                return err(make_error_info("bad float: invalid format",
+                    std::move(src), "here", std::string(
+                    "Hint: valid  : +1.0, -2e-2, 3.141_592_653_589, inf, nan\n"
+                    "Hint: invalid: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0\n")));
+            }
+        }
+        return ok(value_t::integer);
+    }
+    if( ! loc.eof() && loc.current() == '.')
+    {
+        auto src = source_location(region(loc));
+        return err(make_error_info("bad float: integer part is required before decimal point",
+            std::move(src), "missing integer part", std::string(
+            "Hint: valid  : +1.0, -2e-2, 3.141_592_653_589, inf, nan\n"
+            "Hint: invalid: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0\n")
+            ));
+    }
+    if( ! loc.eof() && loc.current() == '_')
+    {
+        auto src = source_location(region(loc));
+        return err(make_error_info("bad number: `_` must be surrounded by digits",
+            std::move(src), "digits required before `_`", std::string(
+            "Hint: valid  : -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755\n"
+            "Hint: invalid: _42, 1__000, 0123\n")
+            ));
+    }
+
+    auto src = source_location(region(loc));
+    return err(make_error_info("bad format: unknown value appeared",
+                std::move(src), "here"));
+}
+
+template<typename TC>
+result<value_t, error_info>
+guess_value_type(const location& loc, const context<TC>& ctx)
+{
+    const auto& sp = ctx.toml_spec();
+    location inner(loc);
+
+    switch(loc.current())
+    {
+        case '"' : {return ok(value_t::string);  }
+        case '\'': {return ok(value_t::string);  }
+        case '[' : {return ok(value_t::array);   }
+        case '{' : {return ok(value_t::table);   }
+        case 't' :
+        {
+            return ok(value_t::boolean);
+        }
+        case 'f' :
+        {
+            return ok(value_t::boolean);
+        }
+        case 'T' : // invalid boolean.
+        {
+            return err(make_syntax_error("toml::parse_value: "
+                "`true` must be in lowercase. "
+                "A string must be surrounded by quotes.",
+                syntax::boolean(sp), inner));
+        }
+        case 'F' :
+        {
+            return err(make_syntax_error("toml::parse_value: "
+                "`false` must be in lowercase. "
+                "A string must be surrounded by quotes.",
+                syntax::boolean(sp), inner));
+        }
+        case 'i' : // inf or string without quotes(syntax error).
+        {
+            if(literal("inf").scan(inner).is_ok())
+            {
+                return ok(value_t::floating);
+            }
+            else
+            {
+                return err(make_syntax_error("toml::parse_value: "
+                    "`inf` must be in lowercase. "
+                    "A string must be surrounded by quotes.",
+                    syntax::floating(sp), inner));
+            }
+        }
+        case 'I' : // Inf or string without quotes(syntax error).
+        {
+            return err(make_syntax_error("toml::parse_value: "
+                "`inf` must be in lowercase. "
+                "A string must be surrounded by quotes.",
+                syntax::floating(sp), inner));
+        }
+        case 'n' : // nan or null-extension
+        {
+            if(sp.ext_null_value)
+            {
+                if(literal("nan").scan(inner).is_ok())
+                {
+                    return ok(value_t::floating);
+                }
+                else if(literal("null").scan(inner).is_ok())
+                {
+                    return ok(value_t::empty);
+                }
+                else
+                {
+                    return err(make_syntax_error("toml::parse_value: "
+                        "Both `nan` and `null` must be in lowercase. "
+                        "A string must be surrounded by quotes.",
+                        syntax::floating(sp), inner));
+                }
+            }
+            else // must be nan.
+            {
+                if(literal("nan").scan(inner).is_ok())
+                {
+                    return ok(value_t::floating);
+                }
+                else
+                {
+                    return err(make_syntax_error("toml::parse_value: "
+                        "`nan` must be in lowercase. "
+                        "A string must be surrounded by quotes.",
+                        syntax::floating(sp), inner));
+                }
+            }
+        }
+        case 'N' : // nan or null-extension
+        {
+            if(sp.ext_null_value)
+            {
+                return err(make_syntax_error("toml::parse_value: "
+                    "Both `nan` and `null` must be in lowercase. "
+                    "A string must be surrounded by quotes.",
+                    syntax::floating(sp), inner));
+            }
+            else
+            {
+                return err(make_syntax_error("toml::parse_value: "
+                    "`nan` must be in lowercase. "
+                    "A string must be surrounded by quotes.",
+                    syntax::floating(sp), inner));
+            }
+        }
+        default  :
+        {
+            return guess_number_type(loc, ctx);
+        }
+    }
+}
+
+template<typename TC>
+result<basic_value<TC>, error_info>
+parse_value(location& loc, context<TC>& ctx)
+{
+    const auto ty_res = guess_value_type(loc, ctx);
+    if(ty_res.is_err())
+    {
+        return err(ty_res.unwrap_err());
+    }
+
+    switch(ty_res.unwrap())
+    {
+        case value_t::empty:
+        {
+            if(ctx.toml_spec().ext_null_value)
+            {
+                return parse_null(loc, ctx);
+            }
+            else
+            {
+                auto src = source_location(region(loc));
+                return err(make_error_info("toml::parse_value: unknown value appeared",
+                            std::move(src), "here"));
+            }
+        }
+        case value_t::boolean        : {return parse_boolean        (loc, ctx);}
+        case value_t::integer        : {return parse_integer        (loc, ctx);}
+        case value_t::floating       : {return parse_floating       (loc, ctx);}
+        case value_t::string         : {return parse_string         (loc, ctx);}
+        case value_t::offset_datetime: {return parse_offset_datetime(loc, ctx);}
+        case value_t::local_datetime : {return parse_local_datetime (loc, ctx);}
+        case value_t::local_date     : {return parse_local_date     (loc, ctx);}
+        case value_t::local_time     : {return parse_local_time     (loc, ctx);}
+        case value_t::array          : {return parse_array          (loc, ctx);}
+        case value_t::table          : {return parse_inline_table   (loc, ctx);}
+        default:
+        {
+            auto src = source_location(region(loc));
+            return err(make_error_info("toml::parse_value: unknown value appeared",
+                        std::move(src), "here"));
+        }
+    }
+}
+
+/* ============================================================================
+ *  _____     _    _
+ * |_   _|_ _| |__| |___
+ *   | |/ _` | '_ \ / -_)
+ *   |_|\__,_|_.__/_\___|
+ */
+
+template<typename TC>
+result<std::pair<std::vector<typename basic_value<TC>::key_type>, region>, error_info>
+parse_table_key(location& loc, context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    auto reg = syntax::std_table(spec).scan(loc);
+    if(!reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_table_key: invalid table key",
+            syntax::std_table(spec), loc));
+    }
+
+    loc = first;
+    loc.advance(); // skip [
+    skip_whitespace(loc, ctx);
+
+    auto keys_res = parse_key(loc, ctx);
+    if(keys_res.is_err())
+    {
+        return err(std::move(keys_res.unwrap_err()));
+    }
+
+    skip_whitespace(loc, ctx);
+    loc.advance(); // ]
+
+    return ok(std::make_pair(std::move(keys_res.unwrap().first), std::move(reg)));
+}
+
+template<typename TC>
+result<std::pair<std::vector<typename basic_value<TC>::key_type>, region>, error_info>
+parse_array_table_key(location& loc, context<TC>& ctx)
+{
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    auto reg = syntax::array_table(spec).scan(loc);
+    if(!reg.is_ok())
+    {
+        return err(make_syntax_error("toml::parse_array_table_key: invalid array-of-tables key",
+            syntax::array_table(spec), loc));
+    }
+
+    loc = first;
+    loc.advance(); // [
+    loc.advance(); // [
+    skip_whitespace(loc, ctx);
+
+    auto keys_res = parse_key(loc, ctx);
+    if(keys_res.is_err())
+    {
+        return err(std::move(keys_res.unwrap_err()));
+    }
+
+    skip_whitespace(loc, ctx);
+    loc.advance(); // ]
+    loc.advance(); // ]
+
+    return ok(std::make_pair(std::move(keys_res.unwrap().first), std::move(reg)));
+}
+
+// called after reading [table.keys] and comments around it.
+// Since table may already contain a subtable ([x.y.z] can be defined before [x]),
+// the table that is being parsed is passed as an argument.
+template<typename TC>
+result<none_t, error_info>
+parse_table(location& loc, context<TC>& ctx, basic_value<TC>& table)
+{
+    assert(table.is_table());
+
+    const auto num_errors = ctx.errors().size();
+    const auto& spec = ctx.toml_spec();
+
+    // clear indent info
+    table.as_table_fmt().indent_type = indent_char::none;
+
+    bool newline_found = true;
+    while( ! loc.eof())
+    {
+        const auto start = loc;
+
+        auto sp = skip_multiline_spacer(loc, ctx, newline_found);
+
+        // if reached to EOF, the table ends here. return.
+        if(loc.eof())
+        {
+            break;
+        }
+        // if next table is comming, return.
+        if(sequence(syntax::ws(spec), character('[')).scan(loc).is_ok())
+        {
+            loc = start;
+            break;
+        }
+        // otherwise, it should be a key-value pair.
+        newline_found = newline_found || (sp.has_value() && sp.value().newline_found);
+        if( ! newline_found)
+        {
+            return err(make_error_info("toml::parse_table: "
+                "newline (LF / CRLF) or EOF is expected",
+                source_location(region(loc)), "here"));
+        }
+        if(sp.has_value() && sp.value().indent_type != indent_char::none)
+        {
+            table.as_table_fmt().indent_type = sp.value().indent_type;
+            table.as_table_fmt().body_indent = sp.value().indent;
+        }
+
+        newline_found = false; // reset
+        if(auto kv_res = parse_key_value_pair(loc, ctx))
+        {
+            auto keys    = std::move(kv_res.unwrap().first.first);
+            auto key_reg = std::move(kv_res.unwrap().first.second);
+            auto val     = std::move(kv_res.unwrap().second);
+
+            if(sp.has_value())
+            {
+                for(const auto& com : sp.value().comments)
+                {
+                    val.comments().push_back(com);
+                }
+            }
+
+            if(auto com_res = parse_comment_line(loc, ctx))
+            {
+                if(auto com_opt = com_res.unwrap())
+                {
+                    val.comments().push_back(com_opt.value());
+                    newline_found = true; // comment includes newline at the end
+                }
+            }
+            else
+            {
+                ctx.report_error(std::move(com_res.unwrap_err()));
+            }
+
+            auto ins_res = insert_value(inserting_value_kind::dotted_keys,
+                    std::addressof(table.as_table()),
+                    keys, std::move(key_reg), std::move(val));
+            if(ins_res.is_err())
+            {
+                ctx.report_error(std::move(ins_res.unwrap_err()));
+            }
+        }
+        else
+        {
+            ctx.report_error(std::move(kv_res.unwrap_err()));
+            skip_key_value_pair(loc, ctx);
+        }
+    }
+
+    if(num_errors < ctx.errors().size())
+    {
+        assert(ctx.has_error()); // already reported
+        return err(ctx.pop_last_error());
+    }
+    return ok();
+}
+
+template<typename TC>
+result<basic_value<TC>, std::vector<error_info>>
+parse_file(location& loc, context<TC>& ctx)
+{
+    using value_type = basic_value<TC>;
+    using table_type = typename value_type::table_type;
+
+    const auto first = loc;
+    const auto& spec = ctx.toml_spec();
+
+    if(loc.eof())
+    {
+        return ok(value_type(table_type(), table_format_info{}, {}, region(loc)));
+    }
+
+    value_type root(table_type(), table_format_info{}, {}, region(loc));
+    root.as_table_fmt().fmt = table_format::multiline;
+    root.as_table_fmt().indent_type = indent_char::none;
+
+    // parse top comment.
+    //
+    // ```toml
+    // # this is a comment for the top-level table.
+    //
+    // key = "the first value"
+    // ```
+    //
+    // ```toml
+    // # this is a comment for "the first value".
+    // key = "the first value"
+    // ```
+    while( ! loc.eof())
+    {
+        if(auto com_res = parse_comment_line(loc, ctx))
+        {
+            if(auto com_opt = com_res.unwrap())
+            {
+                root.comments().push_back(std::move(com_opt.value()));
+            }
+            else // no comment found.
+            {
+                // if it is not an empty line, clear the root comment.
+                if( ! sequence(syntax::ws(spec), syntax::newline(spec)).scan(loc).is_ok())
+                {
+                    loc = first;
+                    root.comments().clear();
+                }
+                break;
+            }
+        }
+        else
+        {
+            ctx.report_error(std::move(com_res.unwrap_err()));
+            skip_comment_block(loc, ctx);
+        }
+    }
+
+    // parse root table
+    {
+        const auto res = parse_table(loc, ctx, root);
+        if(res.is_err())
+        {
+            ctx.report_error(std::move(res.unwrap_err()));
+            skip_until_next_table(loc, ctx);
+        }
+    }
+
+    // parse tables
+
+    while( ! loc.eof())
+    {
+        auto sp = skip_multiline_spacer(loc, ctx, /*newline_found=*/true);
+
+        if(auto key_res = parse_array_table_key(loc, ctx))
+        {
+            auto key = std::move(std::get<0>(key_res.unwrap()));
+            auto reg = std::move(std::get<1>(key_res.unwrap()));
+
+            std::vector<std::string> com;
+            if(sp.has_value())
+            {
+                for(std::size_t i=0; i<sp.value().comments.size(); ++i)
+                {
+                    com.push_back(std::move(sp.value().comments.at(i)));
+                }
+            }
+
+            // [table.def] must be followed by one of
+            // - a comment line
+            // - whitespace + newline
+            // - EOF
+            if(auto com_res = parse_comment_line(loc, ctx))
+            {
+                if(auto com_opt = com_res.unwrap())
+                {
+                    com.push_back(com_opt.value());
+                }
+                else // if there is no comment, ws+newline must exist (or EOF)
+                {
+                    skip_whitespace(loc, ctx);
+                    if( ! loc.eof() && ! syntax::newline(ctx.toml_spec()).scan(loc).is_ok())
+                    {
+                        ctx.report_error(make_syntax_error("toml::parse_file: "
+                            "newline (or EOF) expected",
+                            syntax::newline(ctx.toml_spec()), loc));
+                        skip_until_next_table(loc, ctx);
+                        continue;
+                    }
+                }
+            }
+            else // comment syntax error (rare)
+            {
+                ctx.report_error(com_res.unwrap_err());
+                skip_until_next_table(loc, ctx);
+                continue;
+            }
+
+            table_format_info fmt;
+            fmt.fmt = table_format::multiline;
+            fmt.indent_type = indent_char::none;
+            auto tab = value_type(table_type{}, std::move(fmt), std::move(com), reg);
+
+            auto inserted = insert_value(inserting_value_kind::array_table,
+                std::addressof(root.as_table()),
+                key, std::move(reg), std::move(tab));
+
+            if(inserted.is_err())
+            {
+                ctx.report_error(inserted.unwrap_err());
+
+                // check errors in the table
+                auto tmp = basic_value<TC>(table_type());
+                auto res = parse_table(loc, ctx, tmp);
+                if(res.is_err())
+                {
+                    ctx.report_error(res.unwrap_err());
+                    skip_until_next_table(loc, ctx);
+                }
+                continue;
+            }
+
+            auto tab_ptr = inserted.unwrap();
+            assert(tab_ptr);
+
+            const auto tab_res = parse_table(loc, ctx, *tab_ptr);
+            if(tab_res.is_err())
+            {
+                ctx.report_error(tab_res.unwrap_err());
+                skip_until_next_table(loc, ctx);
+            }
+
+            // parse_table first clears `indent_type`.
+            // to keep header indent info, we must store it later.
+            if(sp.has_value() && sp.value().indent_type != indent_char::none)
+            {
+                tab_ptr->as_table_fmt().indent_type = sp.value().indent_type;
+                tab_ptr->as_table_fmt().name_indent = sp.value().indent;
+            }
+            continue;
+        }
+        if(auto key_res = parse_table_key(loc, ctx))
+        {
+            auto key = std::move(std::get<0>(key_res.unwrap()));
+            auto reg = std::move(std::get<1>(key_res.unwrap()));
+
+            std::vector<std::string> com;
+            if(sp.has_value())
+            {
+                for(std::size_t i=0; i<sp.value().comments.size(); ++i)
+                {
+                    com.push_back(std::move(sp.value().comments.at(i)));
+                }
+            }
+
+            // [table.def] must be followed by one of
+            // - a comment line
+            // - whitespace + newline
+            // - EOF
+            if(auto com_res = parse_comment_line(loc, ctx))
+            {
+                if(auto com_opt = com_res.unwrap())
+                {
+                    com.push_back(com_opt.value());
+                }
+                else // if there is no comment, ws+newline must exist (or EOF)
+                {
+                    skip_whitespace(loc, ctx);
+                    if( ! loc.eof() && ! syntax::newline(ctx.toml_spec()).scan(loc).is_ok())
+                    {
+                        ctx.report_error(make_syntax_error("toml::parse_file: "
+                            "newline (or EOF) expected",
+                            syntax::newline(ctx.toml_spec()), loc));
+                        skip_until_next_table(loc, ctx);
+                        continue;
+                    }
+                }
+            }
+            else // comment syntax error (rare)
+            {
+                ctx.report_error(com_res.unwrap_err());
+                skip_until_next_table(loc, ctx);
+                continue;
+            }
+
+            table_format_info fmt;
+            fmt.fmt = table_format::multiline;
+            fmt.indent_type = indent_char::none;
+            auto tab = value_type(table_type{}, std::move(fmt), std::move(com), reg);
+
+            auto inserted = insert_value(inserting_value_kind::std_table,
+                std::addressof(root.as_table()),
+                key, std::move(reg), std::move(tab));
+
+            if(inserted.is_err())
+            {
+                ctx.report_error(inserted.unwrap_err());
+
+                // check errors in the table
+                auto tmp = basic_value<TC>(table_type());
+                auto res = parse_table(loc, ctx, tmp);
+                if(res.is_err())
+                {
+                    ctx.report_error(res.unwrap_err());
+                    skip_until_next_table(loc, ctx);
+                }
+                continue;
+            }
+
+            auto tab_ptr = inserted.unwrap();
+            assert(tab_ptr);
+
+            const auto tab_res = parse_table(loc, ctx, *tab_ptr);
+            if(tab_res.is_err())
+            {
+                ctx.report_error(tab_res.unwrap_err());
+                skip_until_next_table(loc, ctx);
+            }
+            if(sp.has_value() && sp.value().indent_type != indent_char::none)
+            {
+                tab_ptr->as_table_fmt().indent_type = sp.value().indent_type;
+                tab_ptr->as_table_fmt().name_indent = sp.value().indent;
+            }
+            continue;
+        }
+
+        // does not match array_table nor std_table. report an error.
+        const auto keytop = loc;
+        const auto maybe_array_of_tables = literal("[[").scan(loc).is_ok();
+        loc = keytop;
+
+        if(maybe_array_of_tables)
+        {
+            ctx.report_error(make_syntax_error("toml::parse_file: invalid array-table key",
+                syntax::array_table(spec), loc));
+        }
+        else
+        {
+            ctx.report_error(make_syntax_error("toml::parse_file: invalid table key",
+                syntax::std_table(spec), loc));
+        }
+        skip_until_next_table(loc, ctx);
+    }
+
+    if( ! ctx.errors().empty())
+    {
+        return err(std::move(ctx.errors()));
+    }
+    return ok(std::move(root));
+}
+
+template<typename TC>
+result<basic_value<TC>, std::vector<error_info>>
+parse_impl(std::vector<location::char_type> cs, std::string fname, const spec& s)
+{
+    using value_type = basic_value<TC>;
+    using table_type = typename value_type::table_type;
+
+    // an empty file is a valid toml file.
+    if(cs.empty())
+    {
+        auto src = std::make_shared<std::vector<location::char_type>>(std::move(cs));
+        location loc(std::move(src), std::move(fname));
+        return ok(value_type(table_type(), table_format_info{}, std::vector<std::string>{}, region(loc)));
+    }
+
+    // to simplify parser, add newline at the end if there is no LF.
+    // But, if it has raw CR, the file is invalid (in TOML, CR is not a valid
+    // newline char). if it ends with CR, do not add LF and report it.
+    if(cs.back() != '\n' && cs.back() != '\r')
+    {
+        cs.push_back('\n');
+    }
+
+    auto src = std::make_shared<std::vector<location::char_type>>(std::move(cs));
+
+    location loc(std::move(src), std::move(fname));
+
+    // skip BOM if found
+    if(loc.source()->size() >= 3)
+    {
+        auto first = loc.get_location();
+
+        const auto c0 = loc.current(); loc.advance();
+        const auto c1 = loc.current(); loc.advance();
+        const auto c2 = loc.current(); loc.advance();
+
+        const auto bom_found = (c0 == 0xEF) && (c1 == 0xBB) && (c2 == 0xBF);
+        if( ! bom_found)
+        {
+            loc.set_location(first);
+        }
+    }
+
+    context<TC> ctx(s);
+
+    return parse_file(loc, ctx);
+}
+
+} // detail
+
+// -----------------------------------------------------------------------------
+// parse(byte array)
+
+template<typename TC = type_config>
+result<basic_value<TC>, std::vector<error_info>>
+try_parse(std::vector<unsigned char> content, std::string filename,
+          spec s = spec::default_version())
+{
+    return detail::parse_impl<TC>(std::move(content), std::move(filename), std::move(s));
+}
+template<typename TC = type_config>
+basic_value<TC>
+parse(std::vector<unsigned char> content, std::string filename,
+      spec s = spec::default_version())
+{
+    auto res = try_parse<TC>(std::move(content), std::move(filename), std::move(s));
+    if(res.is_ok())
+    {
+        return res.unwrap();
+    }
+    else
+    {
+        std::string msg;
+        for(const auto& err : res.unwrap_err())
+        {
+            msg += format_error(err);
+        }
+        throw syntax_error(std::move(msg), std::move(res.unwrap_err()));
+    }
+}
+
+// -----------------------------------------------------------------------------
+// parse(istream)
+
+template<typename TC = type_config>
+result<basic_value<TC>, std::vector<error_info>>
+try_parse(std::istream& is, std::string fname = "unknown file", spec s = spec::default_version())
+{
+    const auto beg = is.tellg();
+    is.seekg(0, std::ios::end);
+    const auto end = is.tellg();
+    const auto fsize = end - beg;
+    is.seekg(beg);
+
+    // read whole file as a sequence of char
+    assert(fsize >= 0);
+    std::vector<detail::location::char_type> letters(static_cast<std::size_t>(fsize), '\0');
+    is.read(reinterpret_cast<char*>(letters.data()), static_cast<std::streamsize>(fsize));
+
+    return detail::parse_impl<TC>(std::move(letters), std::move(fname), std::move(s));
+}
+
+template<typename TC = type_config>
+basic_value<TC> parse(std::istream& is, std::string fname = "unknown file", spec s = spec::default_version())
+{
+    auto res = try_parse<TC>(is, std::move(fname), std::move(s));
+    if(res.is_ok())
+    {
+        return res.unwrap();
+    }
+    else
+    {
+        std::string msg;
+        for(const auto& err : res.unwrap_err())
+        {
+            msg += format_error(err);
+        }
+        throw syntax_error(std::move(msg), std::move(res.unwrap_err()));
+    }
+}
+
+// -----------------------------------------------------------------------------
+// parse(filename)
+
+template<typename TC = type_config>
+result<basic_value<TC>, std::vector<error_info>>
+try_parse(std::string fname, spec s = spec::default_version())
+{
+    std::ifstream ifs(fname, std::ios_base::binary);
+    if(!ifs.good())
+    {
+        std::vector<error_info> e;
+        e.push_back(error_info("toml::parse: Error opening file \"" + fname + "\"", {}));
+        return err(std::move(e));
+    }
+    ifs.exceptions(std::ifstream::failbit | std::ifstream::badbit);
+
+    return try_parse<TC>(ifs, std::move(fname), std::move(s));
+}
+
+template<typename TC = type_config>
+basic_value<TC> parse(std::string fname, spec s = spec::default_version())
+{
+    std::ifstream ifs(fname, std::ios_base::binary);
+    if(!ifs.good())
+    {
+        throw file_io_error("toml::parse: error opening file", fname);
+    }
+    ifs.exceptions(std::ifstream::failbit | std::ifstream::badbit);
+
+    return parse<TC>(ifs, std::move(fname), std::move(s));
+}
+
+template<typename TC = type_config, std::size_t N>
+result<basic_value<TC>, std::vector<error_info>>
+try_parse(const char (&fname)[N], spec s = spec::default_version())
+{
+    return try_parse<TC>(std::string(fname), std::move(s));
+}
+
+template<typename TC = type_config, std::size_t N>
+basic_value<TC> parse(const char (&fname)[N], spec s = spec::default_version())
+{
+    return parse<TC>(std::string(fname), std::move(s));
+}
+
+// ----------------------------------------------------------------------------
+// parse_str
+
+template<typename TC = type_config>
+result<basic_value<TC>, std::vector<error_info>>
+try_parse_str(std::string content, spec s = spec::default_version(),
+              cxx::source_location loc = cxx::source_location::current())
+{
+    std::istringstream iss(std::move(content));
+    std::string name("internal string" + cxx::to_string(loc));
+    return try_parse<TC>(iss, std::move(name), std::move(s));
+}
+
+template<typename TC = type_config>
+basic_value<TC> parse_str(std::string content, spec s = spec::default_version(),
+        cxx::source_location loc = cxx::source_location::current())
+{
+    auto res = try_parse_str<TC>(std::move(content), std::move(s), std::move(loc));
+    if(res.is_ok())
+    {
+        return res.unwrap();
+    }
+    else
+    {
+        std::string msg;
+        for(const auto& err : res.unwrap_err())
+        {
+            msg += format_error(err);
+        }
+        throw syntax_error(std::move(msg), std::move(res.unwrap_err()));
+    }
+}
+
+// ----------------------------------------------------------------------------
+// filesystem
+
+#if defined(TOML11_HAS_FILESYSTEM)
+
+template<typename TC = type_config, typename FSPATH>
+cxx::enable_if_t<std::is_same<FSPATH, std::filesystem::path>::value,
+    result<basic_value<TC>, std::vector<error_info>>>
+try_parse(const FSPATH& fpath, spec s = spec::default_version())
+{
+    std::ifstream ifs(fpath, std::ios_base::binary);
+    if(!ifs.good())
+    {
+        std::vector<error_info> e;
+        e.push_back(error_info("toml::parse: Error opening file \"" + fpath.string() + "\"", {}));
+        return err(std::move(e));
+    }
+    ifs.exceptions(std::ifstream::failbit | std::ifstream::badbit);
+
+    return try_parse<TC>(ifs, fpath.string(), std::move(s));
+}
+
+template<typename TC = type_config, typename FSPATH>
+cxx::enable_if_t<std::is_same<FSPATH, std::filesystem::path>::value,
+    basic_value<TC>>
+parse(const FSPATH& fpath, spec s = spec::default_version())
+{
+    std::ifstream ifs(fpath, std::ios_base::binary);
+    if(!ifs.good())
+    {
+        throw file_io_error("toml::parse: error opening file", fpath.string());
+    }
+    ifs.exceptions(std::ifstream::failbit | std::ifstream::badbit);
+
+    return parse<TC>(ifs, fpath.string(), std::move(s));
+}
+#endif
+
+// -----------------------------------------------------------------------------
+// FILE*
+
+template<typename TC = type_config>
+result<basic_value<TC>, std::vector<error_info>>
+try_parse(FILE* fp, std::string filename, spec s = spec::default_version())
+{
+    const long beg = std::ftell(fp);
+    if (beg == -1L)
+    {
+        return err(std::vector<error_info>{error_info(
+            std::string("Failed to access: \"") + filename +
+            "\", errno = " + std::to_string(errno), {}
+        )});
+    }
+
+    const int res_seekend = std::fseek(fp, 0, SEEK_END);
+    if (res_seekend != 0)
+    {
+        return err(std::vector<error_info>{error_info(
+            std::string("Failed to seek: \"") + filename +
+            "\", errno = " + std::to_string(errno), {}
+        )});
+    }
+
+    const long end = std::ftell(fp);
+    if (end == -1L)
+    {
+        return err(std::vector<error_info>{error_info(
+            std::string("Failed to access: \"") + filename +
+            "\", errno = " + std::to_string(errno), {}
+        )});
+    }
+
+    const auto fsize = end - beg;
+
+    const auto res_seekbeg = std::fseek(fp, beg, SEEK_SET);
+    if (res_seekbeg != 0)
+    {
+        return err(std::vector<error_info>{error_info(
+            std::string("Failed to seek: \"") + filename +
+            "\", errno = " + std::to_string(errno), {}
+        )});
+
+    }
+
+    // read whole file as a sequence of char
+    assert(fsize >= 0);
+    std::vector<detail::location::char_type> letters(static_cast<std::size_t>(fsize));
+    const auto actual = std::fread(letters.data(), sizeof(char), static_cast<std::size_t>(fsize), fp);
+    if(actual != static_cast<std::size_t>(fsize))
+    {
+        return err(std::vector<error_info>{error_info(
+            std::string("File size changed: \"") + filename +
+            std::string("\" make sure that FILE* is in binary mode "
+                        "to avoid LF <-> CRLF conversion"), {}
+        )});
+    }
+
+    return detail::parse_impl<TC>(std::move(letters), std::move(filename), std::move(s));
+}
+
+template<typename TC = type_config>
+basic_value<TC>
+parse(FILE* fp, std::string filename, spec s = spec::default_version())
+{
+    const long beg = std::ftell(fp);
+    if (beg == -1L)
+    {
+        throw file_io_error(errno, "Failed to access", filename);
+    }
+
+    const int res_seekend = std::fseek(fp, 0, SEEK_END);
+    if (res_seekend != 0)
+    {
+        throw file_io_error(errno, "Failed to seek", filename);
+    }
+
+    const long end = std::ftell(fp);
+    if (end == -1L)
+    {
+        throw file_io_error(errno, "Failed to access", filename);
+    }
+
+    const auto fsize = end - beg;
+
+    const auto res_seekbeg = std::fseek(fp, beg, SEEK_SET);
+    if (res_seekbeg != 0)
+    {
+        throw file_io_error(errno, "Failed to seek", filename);
+    }
+
+    // read whole file as a sequence of char
+    assert(fsize >= 0);
+    std::vector<detail::location::char_type> letters(static_cast<std::size_t>(fsize));
+    const auto actual = std::fread(letters.data(), sizeof(char), static_cast<std::size_t>(fsize), fp);
+    if(actual != static_cast<std::size_t>(fsize))
+    {
+        throw file_io_error(errno, "File size changed; make sure that "
+            "FILE* is in binary mode to avoid LF <-> CRLF conversion", filename);
+    }
+
+    auto res = detail::parse_impl<TC>(std::move(letters), std::move(filename), std::move(s));
+    if(res.is_ok())
+    {
+        return res.unwrap();
+    }
+    else
+    {
+        std::string msg;
+        for(const auto& err : res.unwrap_err())
+        {
+            msg += format_error(err);
+        }
+        throw syntax_error(std::move(msg), std::move(res.unwrap_err()));
+    }
+}
+
+} // namespace toml
+
+#if defined(TOML11_COMPILE_SOURCES)
+namespace toml
+{
+struct type_config;
+struct ordered_type_config;
+
+extern template result<basic_value<type_config>, std::vector<error_info>> try_parse<type_config>(std::vector<unsigned char>, std::string, spec);
+extern template result<basic_value<type_config>, std::vector<error_info>> try_parse<type_config>(std::istream&, std::string, spec);
+extern template result<basic_value<type_config>, std::vector<error_info>> try_parse<type_config>(std::string, spec);
+extern template result<basic_value<type_config>, std::vector<error_info>> try_parse<type_config>(FILE*, std::string, spec);
+extern template result<basic_value<type_config>, std::vector<error_info>> try_parse_str<type_config>(std::string, spec, cxx::source_location);
+
+extern template basic_value<type_config> parse<type_config>(std::vector<unsigned char>, std::string, spec);
+extern template basic_value<type_config> parse<type_config>(std::istream&, std::string, spec);
+extern template basic_value<type_config> parse<type_config>(std::string, spec);
+extern template basic_value<type_config> parse<type_config>(FILE*, std::string, spec);
+extern template basic_value<type_config> parse_str<type_config>(std::string, spec, cxx::source_location);
+
+extern template result<basic_value<ordered_type_config>, std::vector<error_info>> try_parse<ordered_type_config>(std::vector<unsigned char>, std::string, spec);
+extern template result<basic_value<ordered_type_config>, std::vector<error_info>> try_parse<ordered_type_config>(std::istream&, std::string, spec);
+extern template result<basic_value<ordered_type_config>, std::vector<error_info>> try_parse<ordered_type_config>(std::string, spec);
+extern template result<basic_value<ordered_type_config>, std::vector<error_info>> try_parse<ordered_type_config>(FILE*, std::string, spec);
+extern template result<basic_value<ordered_type_config>, std::vector<error_info>> try_parse_str<ordered_type_config>(std::string, spec, cxx::source_location);
+
+extern template basic_value<ordered_type_config> parse<ordered_type_config>(std::vector<unsigned char>, std::string, spec);
+extern template basic_value<ordered_type_config> parse<ordered_type_config>(std::istream&, std::string, spec);
+extern template basic_value<ordered_type_config> parse<ordered_type_config>(std::string, spec);
+extern template basic_value<ordered_type_config> parse<ordered_type_config>(FILE*, std::string, spec);
+extern template basic_value<ordered_type_config> parse_str<ordered_type_config>(std::string, spec, cxx::source_location);
+
+#if defined(TOML11_HAS_FILESYSTEM)
+extern template cxx::enable_if_t<std::is_same<std::filesystem::path, std::filesystem::path>::value, result<basic_value<type_config>,         std::vector<error_info>>> try_parse<type_config,         std::filesystem::path>(const std::filesystem::path&, spec);
+extern template cxx::enable_if_t<std::is_same<std::filesystem::path, std::filesystem::path>::value, result<basic_value<ordered_type_config>, std::vector<error_info>>> try_parse<ordered_type_config, std::filesystem::path>(const std::filesystem::path&, spec);
+extern template cxx::enable_if_t<std::is_same<std::filesystem::path, std::filesystem::path>::value, basic_value<type_config>                                         > parse    <type_config,         std::filesystem::path>(const std::filesystem::path&, spec);
+extern template cxx::enable_if_t<std::is_same<std::filesystem::path, std::filesystem::path>::value, basic_value<ordered_type_config>                                 > parse    <ordered_type_config, std::filesystem::path>(const std::filesystem::path&, spec);
+#endif // filesystem
+
+} // toml
+#endif // TOML11_COMPILE_SOURCES
+
+#endif // TOML11_PARSER_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/region.hpp b/src/frontend/qt_sdl/toml/toml11/region.hpp
new file mode 100644
index 00000000..b48300c0
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/region.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_REGION_HPP
+#define TOML11_REGION_HPP
+
+#include "fwd/region_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/region_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_REGION_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/result.hpp b/src/frontend/qt_sdl/toml/toml11/result.hpp
new file mode 100644
index 00000000..e847c4b9
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/result.hpp
@@ -0,0 +1,486 @@
+#ifndef TOML11_RESULT_HPP
+#define TOML11_RESULT_HPP
+
+#include "compat.hpp"
+#include "exception.hpp"
+
+#include <ostream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include <cassert>
+
+namespace toml
+{
+
+struct bad_result_access final : public ::toml::exception
+{
+  public:
+    explicit bad_result_access(std::string what_arg)
+        : what_(std::move(what_arg))
+    {}
+    ~bad_result_access() noexcept override = default;
+    const char* what() const noexcept override {return what_.c_str();}
+
+  private:
+    std::string what_;
+};
+
+// -----------------------------------------------------------------------------
+
+template<typename T>
+struct success
+{
+    static_assert( ! std::is_same<T, void>::value, "");
+
+    using value_type = T;
+
+    explicit success(value_type v)
+        noexcept(std::is_nothrow_move_constructible<value_type>::value)
+        : value(std::move(v))
+    {}
+
+    template<typename U, cxx::enable_if_t<
+        std::is_convertible<cxx::remove_cvref_t<U>, T>::value,
+        std::nullptr_t> = nullptr>
+    explicit success(U&& v): value(std::forward<U>(v)) {}
+
+    template<typename U>
+    explicit success(success<U> v): value(std::move(v.value)) {}
+
+    ~success() = default;
+    success(const success&) = default;
+    success(success&&)      = default;
+    success& operator=(const success&) = default;
+    success& operator=(success&&)      = default;
+
+    value_type&       get()       noexcept {return value;}
+    value_type const& get() const noexcept {return value;}
+
+  private:
+
+    value_type value;
+};
+
+template<typename T>
+struct success<std::reference_wrapper<T>>
+{
+    static_assert( ! std::is_same<T, void>::value, "");
+
+    using value_type = T;
+
+    explicit success(std::reference_wrapper<value_type> v) noexcept
+        : value(std::move(v))
+    {}
+
+    ~success() = default;
+    success(const success&) = default;
+    success(success&&)      = default;
+    success& operator=(const success&) = default;
+    success& operator=(success&&)      = default;
+
+    value_type&       get()       noexcept {return value.get();}
+    value_type const& get() const noexcept {return value.get();}
+
+  private:
+
+    std::reference_wrapper<value_type> value;
+};
+
+template<typename T>
+success<typename std::decay<T>::type> ok(T&& v)
+{
+    return success<typename std::decay<T>::type>(std::forward<T>(v));
+}
+template<std::size_t N>
+success<std::string> ok(const char (&literal)[N])
+{
+    return success<std::string>(std::string(literal));
+}
+
+// -----------------------------------------------------------------------------
+
+template<typename T>
+struct failure
+{
+    using value_type = T;
+
+    explicit failure(value_type v)
+        noexcept(std::is_nothrow_move_constructible<value_type>::value)
+        : value(std::move(v))
+    {}
+
+    template<typename U, cxx::enable_if_t<
+        std::is_convertible<cxx::remove_cvref_t<U>, T>::value,
+        std::nullptr_t> = nullptr>
+    explicit failure(U&& v): value(std::forward<U>(v)) {}
+
+    template<typename U>
+    explicit failure(failure<U> v): value(std::move(v.value)) {}
+
+    ~failure() = default;
+    failure(const failure&) = default;
+    failure(failure&&)      = default;
+    failure& operator=(const failure&) = default;
+    failure& operator=(failure&&)      = default;
+
+    value_type&       get()       noexcept {return value;}
+    value_type const& get() const noexcept {return value;}
+
+  private:
+
+    value_type value;
+};
+
+template<typename T>
+struct failure<std::reference_wrapper<T>>
+{
+    using value_type = T;
+
+    explicit failure(std::reference_wrapper<value_type> v) noexcept
+        : value(std::move(v))
+    {}
+
+    ~failure() = default;
+    failure(const failure&) = default;
+    failure(failure&&)      = default;
+    failure& operator=(const failure&) = default;
+    failure& operator=(failure&&)      = default;
+
+    value_type&       get()       noexcept {return value.get();}
+    value_type const& get() const noexcept {return value.get();}
+
+  private:
+
+    std::reference_wrapper<value_type> value;
+};
+
+template<typename T>
+failure<typename std::decay<T>::type> err(T&& v)
+{
+    return failure<typename std::decay<T>::type>(std::forward<T>(v));
+}
+
+template<std::size_t N>
+failure<std::string> err(const char (&literal)[N])
+{
+    return failure<std::string>(std::string(literal));
+}
+
+/* ============================================================================
+ *                  _ _
+ *  _ _ ___ ____  _| | |_
+ * | '_/ -_|_-< || | |  _|
+ * |_| \___/__/\_,_|_|\__|
+ */
+
+template<typename T, typename E>
+struct result
+{
+    using success_type = success<T>;
+    using failure_type = failure<E>;
+    using value_type = typename success_type::value_type;
+    using error_type = typename failure_type::value_type;
+
+    result(success_type s): is_ok_(true),  succ_(std::move(s)) {}
+    result(failure_type f): is_ok_(false), fail_(std::move(f)) {}
+
+    template<typename U, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<U>, value_type>>,
+            std::is_convertible<cxx::remove_cvref_t<U>, value_type>
+        >::value, std::nullptr_t> = nullptr>
+    result(success<U> s): is_ok_(true),  succ_(std::move(s.value)) {}
+
+    template<typename U, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<U>, error_type>>,
+            std::is_convertible<cxx::remove_cvref_t<U>, error_type>
+        >::value, std::nullptr_t> = nullptr>
+    result(failure<U> f): is_ok_(false), fail_(std::move(f.value)) {}
+
+    result& operator=(success_type s)
+    {
+        this->cleanup();
+        this->is_ok_ = true;
+        auto tmp = ::new(std::addressof(this->succ_)) success_type(std::move(s));
+        assert(tmp == std::addressof(this->succ_));
+        (void)tmp;
+        return *this;
+    }
+    result& operator=(failure_type f)
+    {
+        this->cleanup();
+        this->is_ok_ = false;
+        auto tmp = ::new(std::addressof(this->fail_)) failure_type(std::move(f));
+        assert(tmp == std::addressof(this->fail_));
+        (void)tmp;
+        return *this;
+    }
+
+    template<typename U>
+    result& operator=(success<U> s)
+    {
+        this->cleanup();
+        this->is_ok_ = true;
+        auto tmp = ::new(std::addressof(this->succ_)) success_type(std::move(s.value));
+        assert(tmp == std::addressof(this->succ_));
+        (void)tmp;
+        return *this;
+    }
+    template<typename U>
+    result& operator=(failure<U> f)
+    {
+        this->cleanup();
+        this->is_ok_ = false;
+        auto tmp = ::new(std::addressof(this->fail_)) failure_type(std::move(f.value));
+        assert(tmp == std::addressof(this->fail_));
+        (void)tmp;
+        return *this;
+    }
+
+    ~result() noexcept {this->cleanup();}
+
+    result(const result& other): is_ok_(other.is_ok())
+    {
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ_)) success_type(other.succ_);
+            assert(tmp == std::addressof(this->succ_));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail_)) failure_type(other.fail_);
+            assert(tmp == std::addressof(this->fail_));
+            (void)tmp;
+        }
+    }
+    result(result&& other): is_ok_(other.is_ok())
+    {
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ_)) success_type(std::move(other.succ_));
+            assert(tmp == std::addressof(this->succ_));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail_)) failure_type(std::move(other.fail_));
+            assert(tmp == std::addressof(this->fail_));
+            (void)tmp;
+        }
+    }
+
+    result& operator=(const result& other)
+    {
+        this->cleanup();
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ_)) success_type(other.succ_);
+            assert(tmp == std::addressof(this->succ_));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail_)) failure_type(other.fail_);
+            assert(tmp == std::addressof(this->fail_));
+            (void)tmp;
+        }
+        is_ok_ = other.is_ok();
+        return *this;
+    }
+    result& operator=(result&& other)
+    {
+        this->cleanup();
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ_)) success_type(std::move(other.succ_));
+            assert(tmp == std::addressof(this->succ_));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail_)) failure_type(std::move(other.fail_));
+            assert(tmp == std::addressof(this->fail_));
+            (void)tmp;
+        }
+        is_ok_ = other.is_ok();
+        return *this;
+    }
+
+    template<typename U, typename F, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<U>, value_type>>,
+            cxx::negation<std::is_same<cxx::remove_cvref_t<F>, error_type>>,
+            std::is_convertible<cxx::remove_cvref_t<U>, value_type>,
+            std::is_convertible<cxx::remove_cvref_t<F>, error_type>
+        >::value, std::nullptr_t> = nullptr>
+    result(result<U, F> other): is_ok_(other.is_ok())
+    {
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ_)) success_type(std::move(other.as_ok()));
+            assert(tmp == std::addressof(this->succ_));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail_)) failure_type(std::move(other.as_err()));
+            assert(tmp == std::addressof(this->fail_));
+            (void)tmp;
+        }
+    }
+
+    template<typename U, typename F, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<U>, value_type>>,
+            cxx::negation<std::is_same<cxx::remove_cvref_t<F>, error_type>>,
+            std::is_convertible<cxx::remove_cvref_t<U>, value_type>,
+            std::is_convertible<cxx::remove_cvref_t<F>, error_type>
+        >::value, std::nullptr_t> = nullptr>
+    result& operator=(result<U, F> other)
+    {
+        this->cleanup();
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ_)) success_type(std::move(other.as_ok()));
+            assert(tmp == std::addressof(this->succ_));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail_)) failure_type(std::move(other.as_err()));
+            assert(tmp == std::addressof(this->fail_));
+            (void)tmp;
+        }
+        is_ok_ = other.is_ok();
+        return *this;
+    }
+
+    bool is_ok()  const noexcept {return is_ok_;}
+    bool is_err() const noexcept {return !is_ok_;}
+
+    explicit operator bool() const noexcept {return is_ok_;}
+
+    value_type& unwrap(cxx::source_location loc = cxx::source_location::current())
+    {
+        if(this->is_err())
+        {
+            throw bad_result_access("toml::result: bad unwrap" + cxx::to_string(loc));
+        }
+        return this->succ_.get();
+    }
+    value_type const& unwrap(cxx::source_location loc = cxx::source_location::current()) const
+    {
+        if(this->is_err())
+        {
+            throw bad_result_access("toml::result: bad unwrap" + cxx::to_string(loc));
+        }
+        return this->succ_.get();
+    }
+
+    value_type& unwrap_or(value_type& opt) noexcept
+    {
+        if(this->is_err()) {return opt;}
+        return this->succ_.get();
+    }
+    value_type const& unwrap_or(value_type const& opt) const noexcept
+    {
+        if(this->is_err()) {return opt;}
+        return this->succ_.get();
+    }
+
+    error_type& unwrap_err(cxx::source_location loc = cxx::source_location::current())
+    {
+        if(this->is_ok())
+        {
+            throw bad_result_access("toml::result: bad unwrap_err" + cxx::to_string(loc));
+        }
+        return this->fail_.get();
+    }
+    error_type const& unwrap_err(cxx::source_location loc = cxx::source_location::current()) const
+    {
+        if(this->is_ok())
+        {
+            throw bad_result_access("toml::result: bad unwrap_err" + cxx::to_string(loc));
+        }
+        return this->fail_.get();
+    }
+
+    value_type& as_ok() noexcept
+    {
+        assert(this->is_ok());
+        return this->succ_.get();
+    }
+    value_type const& as_ok() const noexcept
+    {
+        assert(this->is_ok());
+        return this->succ_.get();
+    }
+
+    error_type& as_err() noexcept
+    {
+        assert(this->is_err());
+        return this->fail_.get();
+    }
+    error_type const& as_err() const noexcept
+    {
+        assert(this->is_err());
+        return this->fail_.get();
+    }
+
+  private:
+
+    void cleanup() noexcept
+    {
+#if defined(__GNUC__) && ! defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wduplicated-branches"
+#endif
+
+        if(this->is_ok_) {this->succ_.~success_type();}
+        else             {this->fail_.~failure_type();}
+
+#if defined(__GNUC__) && ! defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+        return;
+    }
+
+  private:
+
+    bool      is_ok_;
+    union
+    {
+        success_type succ_;
+        failure_type fail_;
+    };
+};
+
+// ----------------------------------------------------------------------------
+
+namespace detail
+{
+struct none_t {};
+inline bool operator==(const none_t&, const none_t&) noexcept {return true;}
+inline bool operator!=(const none_t&, const none_t&) noexcept {return false;}
+inline bool operator< (const none_t&, const none_t&) noexcept {return false;}
+inline bool operator<=(const none_t&, const none_t&) noexcept {return true;}
+inline bool operator> (const none_t&, const none_t&) noexcept {return false;}
+inline bool operator>=(const none_t&, const none_t&) noexcept {return true;}
+inline std::ostream& operator<<(std::ostream& os, const none_t&)
+{
+    os << "none";
+    return os;
+}
+} // detail
+
+inline success<detail::none_t> ok() noexcept
+{
+    return success<detail::none_t>(detail::none_t{});
+}
+inline failure<detail::none_t> err() noexcept
+{
+    return failure<detail::none_t>(detail::none_t{});
+}
+
+} // toml
+#endif // TOML11_RESULT_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/scanner.hpp b/src/frontend/qt_sdl/toml/toml11/scanner.hpp
new file mode 100644
index 00000000..85812d9a
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/scanner.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_SCANNER_HPP
+#define TOML11_SCANNER_HPP
+
+#include "fwd/scanner_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/scanner_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_SCANNER_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/serializer.hpp b/src/frontend/qt_sdl/toml/toml11/serializer.hpp
new file mode 100644
index 00000000..a3f148e2
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/serializer.hpp
@@ -0,0 +1,1275 @@
+#ifndef TOML11_SERIALIZER_HPP
+#define TOML11_SERIALIZER_HPP
+
+#include "comments.hpp"
+#include "error_info.hpp"
+#include "exception.hpp"
+#include "source_location.hpp"
+#include "spec.hpp"
+#include "syntax.hpp"
+#include "types.hpp"
+#include "utility.hpp"
+#include "value.hpp"
+
+#include <iomanip>
+#include <iterator>
+#include <sstream>
+
+#include <cmath>
+#include <cstdio>
+
+namespace toml
+{
+
+struct serialization_error final : public ::toml::exception
+{
+  public:
+    explicit serialization_error(std::string what_arg, source_location loc)
+        : what_(std::move(what_arg)), loc_(std::move(loc))
+    {}
+    ~serialization_error() noexcept override = default;
+
+    const char* what() const noexcept override {return what_.c_str();}
+    source_location const& location() const noexcept {return loc_;}
+
+  private:
+    std::string what_;
+    source_location loc_;
+};
+
+namespace detail
+{
+template<typename TC>
+class serializer
+{
+  public:
+
+    using value_type           = basic_value<TC>;
+
+    using key_type             = typename value_type::key_type            ;
+    using comment_type         = typename value_type::comment_type        ;
+    using boolean_type         = typename value_type::boolean_type        ;
+    using integer_type         = typename value_type::integer_type        ;
+    using floating_type        = typename value_type::floating_type       ;
+    using string_type          = typename value_type::string_type         ;
+    using local_time_type      = typename value_type::local_time_type     ;
+    using local_date_type      = typename value_type::local_date_type     ;
+    using local_datetime_type  = typename value_type::local_datetime_type ;
+    using offset_datetime_type = typename value_type::offset_datetime_type;
+    using array_type           = typename value_type::array_type          ;
+    using table_type           = typename value_type::table_type          ;
+
+    using char_type            = typename string_type::value_type;
+
+  public:
+
+    explicit serializer(const spec& sp)
+        : spec_(sp), force_inline_(false), current_indent_(0)
+    {}
+
+    string_type operator()(const std::vector<key_type>& ks, const value_type& v)
+    {
+        for(const auto& k : ks)
+        {
+            this->keys_.push_back(k);
+        }
+        return (*this)(v);
+    }
+
+    string_type operator()(const key_type& k, const value_type& v)
+    {
+        this->keys_.push_back(k);
+        return (*this)(v);
+    }
+
+    string_type operator()(const value_type& v)
+    {
+        switch(v.type())
+        {
+            case value_t::boolean        : {return (*this)(v.as_boolean        (), v.as_boolean_fmt        (), v.location());}
+            case value_t::integer        : {return (*this)(v.as_integer        (), v.as_integer_fmt        (), v.location());}
+            case value_t::floating       : {return (*this)(v.as_floating       (), v.as_floating_fmt       (), v.location());}
+            case value_t::string         : {return (*this)(v.as_string         (), v.as_string_fmt         (), v.location());}
+            case value_t::offset_datetime: {return (*this)(v.as_offset_datetime(), v.as_offset_datetime_fmt(), v.location());}
+            case value_t::local_datetime : {return (*this)(v.as_local_datetime (), v.as_local_datetime_fmt (), v.location());}
+            case value_t::local_date     : {return (*this)(v.as_local_date     (), v.as_local_date_fmt     (), v.location());}
+            case value_t::local_time     : {return (*this)(v.as_local_time     (), v.as_local_time_fmt     (), v.location());}
+            case value_t::array          :
+            {
+                return (*this)(v.as_array(), v.as_array_fmt(), v.comments(), v.location());
+            }
+            case value_t::table          :
+            {
+                string_type retval;
+                if(this->keys_.empty()) // it might be the root table. emit comments here.
+                {
+                    retval += format_comments(v.comments(), v.as_table_fmt().indent_type);
+                }
+                if( ! retval.empty()) // we have comment.
+                {
+                    retval += char_type('\n');
+                }
+
+                retval += (*this)(v.as_table(), v.as_table_fmt(), v.comments(), v.location());
+                return retval;
+            }
+            case value_t::empty:
+            {
+                if(this->spec_.ext_null_value)
+                {
+                    return string_conv<string_type>("null");
+                }
+                break;
+            }
+            default:
+            {
+                break;
+            }
+        }
+        throw serialization_error(format_error(
+            "[error] toml::serializer: toml::basic_value "
+            "does not have any valid type.", v.location(), "here"), v.location());
+    }
+
+  private:
+
+    string_type operator()(const boolean_type& b, const boolean_format_info&, const source_location&) // {{{
+    {
+        if(b)
+        {
+            return string_conv<string_type>("true");
+        }
+        else
+        {
+            return string_conv<string_type>("false");
+        }
+    } // }}}
+
+    string_type operator()(const integer_type i, const integer_format_info& fmt, const source_location& loc) // {{{
+    {
+        std::ostringstream oss;
+        this->set_locale(oss);
+
+        const auto insert_spacer = [&fmt](std::string s) -> std::string {
+            if(fmt.spacer == 0) {return s;}
+
+            std::string sign;
+            if( ! s.empty() && (s.at(0) == '+' || s.at(0) == '-'))
+            {
+                sign += s.at(0);
+                s.erase(s.begin());
+            }
+
+            std::string spaced;
+            std::size_t counter = 0;
+            for(auto iter = s.rbegin(); iter != s.rend(); ++iter)
+            {
+                if(counter != 0 && counter % fmt.spacer == 0)
+                {
+                    spaced += '_';
+                }
+                spaced += *iter;
+                counter += 1;
+            }
+            if(!spaced.empty() && spaced.back() == '_') {spaced.pop_back();}
+
+            s.clear();
+            std::copy(spaced.rbegin(), spaced.rend(), std::back_inserter(s));
+            return sign + s;
+        };
+
+        std::string retval;
+        if(fmt.fmt == integer_format::dec)
+        {
+            oss << std::setw(static_cast<int>(fmt.width)) << std::dec << i;
+            retval = insert_spacer(oss.str());
+
+            if(this->spec_.ext_num_suffix && ! fmt.suffix.empty())
+            {
+                retval += '_';
+                retval += fmt.suffix;
+            }
+        }
+        else
+        {
+            if(i < 0)
+            {
+                throw serialization_error(format_error("binary, octal, hexadecimal "
+                    "integer does not allow negative value", loc, "here"), loc);
+            }
+            switch(fmt.fmt)
+            {
+                case integer_format::hex:
+                {
+                    oss << std::noshowbase
+                        << std::setw(static_cast<int>(fmt.width))
+                        << std::setfill('0')
+                        << std::hex;
+                    if(fmt.uppercase)
+                    {
+                        oss << std::uppercase;
+                    }
+                    else
+                    {
+                        oss << std::nouppercase;
+                    }
+                    oss << i;
+                    retval = std::string("0x") + insert_spacer(oss.str());
+                    break;
+                }
+                case integer_format::oct:
+                {
+                    oss << std::setw(static_cast<int>(fmt.width)) << std::setfill('0') << std::oct << i;
+                    retval = std::string("0o") + insert_spacer(oss.str());
+                    break;
+                }
+                case integer_format::bin:
+                {
+                    integer_type x{i};
+                    std::string tmp;
+                    std::size_t bits(0);
+                    while(x != 0)
+                    {
+                        if(fmt.spacer != 0)
+                        {
+                            if(bits != 0 && (bits % fmt.spacer) == 0) {tmp += '_';}
+                        }
+                        if(x % 2 == 1) { tmp += '1'; } else { tmp += '0'; }
+                        x >>= 1;
+                        bits += 1;
+                    }
+                    for(; bits < fmt.width; ++bits)
+                    {
+                        if(fmt.spacer != 0)
+                        {
+                            if(bits != 0 && (bits % fmt.spacer) == 0) {tmp += '_';}
+                        }
+                        tmp += '0';
+                    }
+                    for(auto iter = tmp.rbegin(); iter != tmp.rend(); ++iter)
+                    {
+                        oss << *iter;
+                    }
+                    retval = std::string("0b") + oss.str();
+                    break;
+                }
+                default:
+                {
+                    throw serialization_error(format_error(
+                        "none of dec, hex, oct, bin: " + to_string(fmt.fmt),
+                        loc, "here"), loc);
+                }
+            }
+        }
+        return string_conv<string_type>(retval);
+    } // }}}
+
+    string_type operator()(const floating_type f, const floating_format_info& fmt, const source_location&) // {{{
+    {
+        using std::isnan;
+        using std::isinf;
+        using std::signbit;
+
+        std::ostringstream oss;
+        this->set_locale(oss);
+
+        if(isnan(f))
+        {
+            if(signbit(f))
+            {
+                oss << '-';
+            }
+            oss << "nan";
+            if(this->spec_.ext_num_suffix && ! fmt.suffix.empty())
+            {
+                oss << '_';
+                oss << fmt.suffix;
+            }
+            return string_conv<string_type>(oss.str());
+        }
+
+        if(isinf(f))
+        {
+            if(signbit(f))
+            {
+                oss << '-';
+            }
+            oss << "inf";
+            if(this->spec_.ext_num_suffix && ! fmt.suffix.empty())
+            {
+                oss << '_';
+                oss << fmt.suffix;
+            }
+            return string_conv<string_type>(oss.str());
+        }
+
+        switch(fmt.fmt)
+        {
+            case floating_format::defaultfloat:
+            {
+                if(fmt.prec != 0)
+                {
+                    oss << std::setprecision(static_cast<int>(fmt.prec));
+                }
+                oss << f;
+                // since defaultfloat may omit point, we need to add it
+                std::string s = oss.str();
+                if (s.find('.') == std::string::npos &&
+                    s.find('e') == std::string::npos &&
+                    s.find('E') == std::string::npos )
+                {
+                    s += ".0";
+                }
+                if(this->spec_.ext_num_suffix && ! fmt.suffix.empty())
+                {
+                    s += '_';
+                    s += fmt.suffix;
+                }
+                return string_conv<string_type>(s);
+            }
+            case floating_format::fixed:
+            {
+                if(fmt.prec != 0)
+                {
+                    oss << std::setprecision(static_cast<int>(fmt.prec));
+                }
+                oss << std::fixed << f;
+                if(this->spec_.ext_num_suffix && ! fmt.suffix.empty())
+                {
+                    oss << '_' << fmt.suffix;
+                }
+                return string_conv<string_type>(oss.str());
+            }
+            case floating_format::scientific:
+            {
+                if(fmt.prec != 0)
+                {
+                    oss << std::setprecision(static_cast<int>(fmt.prec));
+                }
+                oss << std::scientific << f;
+                if(this->spec_.ext_num_suffix && ! fmt.suffix.empty())
+                {
+                    oss << '_' << fmt.suffix;
+                }
+                return string_conv<string_type>(oss.str());
+            }
+            case floating_format::hex:
+            {
+                if(this->spec_.ext_hex_float)
+                {
+                    oss << std::hexfloat << f;
+                    // suffix is only for decimal numbers.
+                    return string_conv<string_type>(oss.str());
+                }
+                else // no hex allowed. output with max precision.
+                {
+                    oss << std::setprecision(std::numeric_limits<floating_type>::max_digits10)
+                        << std::scientific << f;
+                    // suffix is only for decimal numbers.
+                    return string_conv<string_type>(oss.str());
+                }
+            }
+            default:
+            {
+                if(this->spec_.ext_num_suffix && ! fmt.suffix.empty())
+                {
+                    oss << '_' << fmt.suffix;
+                }
+                return string_conv<string_type>(oss.str());
+            }
+        }
+    } // }}}
+
+    string_type operator()(string_type s, const string_format_info& fmt, const source_location& loc) // {{{
+    {
+        string_type retval;
+        switch(fmt.fmt)
+        {
+            case string_format::basic:
+            {
+                retval += char_type('"');
+                retval += this->escape_basic_string(s);
+                retval += char_type('"');
+                return retval;
+            }
+            case string_format::literal:
+            {
+                if(std::find(s.begin(), s.end(), char_type('\n')) != s.end())
+                {
+                    throw serialization_error(format_error("toml::serializer: "
+                        "(non-multiline) literal string cannot have a newline",
+                        loc, "here"), loc);
+                }
+                retval += char_type('\'');
+                retval += s;
+                retval += char_type('\'');
+                return retval;
+            }
+            case string_format::multiline_basic:
+            {
+                retval += string_conv<string_type>("\"\"\"");
+                if(fmt.start_with_newline)
+                {
+                    retval += char_type('\n');
+                }
+
+                retval += this->escape_ml_basic_string(s);
+
+                retval += string_conv<string_type>("\"\"\"");
+                return retval;
+            }
+            case string_format::multiline_literal:
+            {
+                retval += string_conv<string_type>("'''");
+                if(fmt.start_with_newline)
+                {
+                    retval += char_type('\n');
+                }
+                retval += s;
+                retval += string_conv<string_type>("'''");
+                return retval;
+            }
+            default:
+            {
+                throw serialization_error(format_error(
+                    "[error] toml::serializer::operator()(string): "
+                    "invalid string_format value", loc, "here"), loc);
+            }
+        }
+    } // }}}
+
+    string_type operator()(const local_date_type& d, const local_date_format_info&, const source_location&) // {{{
+    {
+        std::ostringstream oss;
+        oss << d;
+        return string_conv<string_type>(oss.str());
+    } // }}}
+
+    string_type operator()(const local_time_type& t, const local_time_format_info& fmt, const source_location&) // {{{
+    {
+        return this->format_local_time(t, fmt.has_seconds, fmt.subsecond_precision);
+    } // }}}
+
+    string_type operator()(const local_datetime_type& dt, const local_datetime_format_info& fmt, const source_location&) // {{{
+    {
+        std::ostringstream oss;
+        oss << dt.date;
+        switch(fmt.delimiter)
+        {
+            case datetime_delimiter_kind::upper_T: { oss << 'T'; break; }
+            case datetime_delimiter_kind::lower_t: { oss << 't'; break; }
+            case datetime_delimiter_kind::space:   { oss << ' '; break; }
+            default:                               { oss << 'T'; break; }
+        }
+        return string_conv<string_type>(oss.str()) +
+            this->format_local_time(dt.time, fmt.has_seconds, fmt.subsecond_precision);
+    } // }}}
+
+    string_type operator()(const offset_datetime_type& odt, const offset_datetime_format_info& fmt, const source_location&) // {{{
+    {
+        std::ostringstream oss;
+        oss << odt.date;
+        switch(fmt.delimiter)
+        {
+            case datetime_delimiter_kind::upper_T: { oss << 'T'; break; }
+            case datetime_delimiter_kind::lower_t: { oss << 't'; break; }
+            case datetime_delimiter_kind::space:   { oss << ' '; break; }
+            default:                               { oss << 'T'; break; }
+        }
+        oss << string_conv<std::string>(this->format_local_time(odt.time, fmt.has_seconds, fmt.subsecond_precision));
+        oss << odt.offset;
+        return string_conv<string_type>(oss.str());
+    } // }}}
+
+    string_type operator()(const array_type& a, const array_format_info& fmt, const comment_type& com, const source_location& loc) // {{{
+    {
+        array_format f = fmt.fmt;
+        if(fmt.fmt == array_format::default_format)
+        {
+            // [[in.this.form]], you cannot add a comment to the array itself
+            // (but you can add a comment to each table).
+            // To keep comments, we need to avoid multiline array-of-tables
+            // if array itself has a comment.
+            if( ! this->keys_.empty() &&
+                ! a.empty() &&
+                com.empty() &&
+                std::all_of(a.begin(), a.end(), [](const value_type& e) {return e.is_table();}))
+            {
+                f = array_format::array_of_tables;
+            }
+            else
+            {
+                f = array_format::oneline;
+
+                // check if it becomes long
+                std::size_t approx_len = 0;
+                for(const auto& e : a)
+                {
+                    // have a comment. cannot be inlined
+                    if( ! e.comments().empty())
+                    {
+                        f = array_format::multiline;
+                        break;
+                    }
+                    // possibly long types ...
+                    if(e.is_array() || e.is_table() || e.is_offset_datetime() || e.is_local_datetime())
+                    {
+                        f = array_format::multiline;
+                        break;
+                    }
+                    else if(e.is_boolean())
+                    {
+                        approx_len += (*this)(e.as_boolean(), e.as_boolean_fmt(), e.location()).size();
+                    }
+                    else if(e.is_integer())
+                    {
+                        approx_len += (*this)(e.as_integer(), e.as_integer_fmt(), e.location()).size();
+                    }
+                    else if(e.is_floating())
+                    {
+                        approx_len += (*this)(e.as_floating(), e.as_floating_fmt(), e.location()).size();
+                    }
+                    else if(e.is_string())
+                    {
+                        if(e.as_string_fmt().fmt == string_format::multiline_basic ||
+                           e.as_string_fmt().fmt == string_format::multiline_literal)
+                        {
+                            f = array_format::multiline;
+                            break;
+                        }
+                        approx_len += 2 + (*this)(e.as_string(), e.as_string_fmt(), e.location()).size();
+                    }
+                    else if(e.is_local_date())
+                    {
+                        approx_len += 10; // 1234-56-78
+                    }
+                    else if(e.is_local_time())
+                    {
+                        approx_len += 15; // 12:34:56.789012
+                    }
+
+                    if(approx_len > 60) // key, ` = `, `[...]` < 80
+                    {
+                        f = array_format::multiline;
+                        break;
+                    }
+                    approx_len += 2; // `, `
+                }
+            }
+        }
+        if(this->force_inline_ && f == array_format::array_of_tables)
+        {
+            f = array_format::multiline;
+        }
+        if(a.empty() && f == array_format::array_of_tables)
+        {
+            f = array_format::oneline;
+        }
+
+        // --------------------------------------------------------------------
+
+        if(f == array_format::array_of_tables)
+        {
+            if(this->keys_.empty())
+            {
+                throw serialization_error("array of table must have its key. "
+                        "use format(key, v)", loc);
+            }
+            string_type retval;
+            for(const auto& e : a)
+            {
+                assert(e.is_table());
+
+                this->current_indent_ += e.as_table_fmt().name_indent;
+                retval += this->format_comments(e.comments(), e.as_table_fmt().indent_type);
+                retval += this->format_indent(e.as_table_fmt().indent_type);
+                this->current_indent_ -= e.as_table_fmt().name_indent;
+
+                retval += string_conv<string_type>("[[");
+                retval += this->format_keys(this->keys_).value();
+                retval += string_conv<string_type>("]]\n");
+
+                retval += this->format_ml_table(e.as_table(), e.as_table_fmt());
+            }
+            return retval;
+        }
+        else if(f == array_format::oneline)
+        {
+            // ignore comments. we cannot emit comments
+            string_type retval;
+            retval += char_type('[');
+            for(const auto& e : a)
+            {
+                this->force_inline_ = true;
+                retval += (*this)(e);
+                retval += string_conv<string_type>(", ");
+            }
+            if( ! a.empty())
+            {
+                retval.pop_back(); // ` `
+                retval.pop_back(); // `,`
+            }
+            retval += char_type(']');
+            this->force_inline_ = false;
+            return retval;
+        }
+        else
+        {
+            assert(f == array_format::multiline);
+
+            string_type retval;
+            retval += string_conv<string_type>("[\n");
+
+            for(const auto& e : a)
+            {
+                this->current_indent_ += fmt.body_indent;
+                retval += this->format_comments(e.comments(), fmt.indent_type);
+                retval += this->format_indent(fmt.indent_type);
+                this->current_indent_ -= fmt.body_indent;
+
+                this->force_inline_ = true;
+                retval += (*this)(e);
+                retval += string_conv<string_type>(",\n");
+            }
+            this->force_inline_ = false;
+
+            this->current_indent_ += fmt.closing_indent;
+            retval += this->format_indent(fmt.indent_type);
+            this->current_indent_ -= fmt.closing_indent;
+
+            retval += char_type(']');
+            return retval;
+        }
+    } // }}}
+
+    string_type operator()(const table_type& t, const table_format_info& fmt, const comment_type& com, const source_location& loc) // {{{
+    {
+        if(this->force_inline_)
+        {
+            if(fmt.fmt == table_format::multiline_oneline)
+            {
+                return this->format_ml_inline_table(t, fmt);
+            }
+            else
+            {
+                return this->format_inline_table(t, fmt);
+            }
+        }
+        else
+        {
+            if(fmt.fmt == table_format::multiline)
+            {
+                string_type retval;
+                // comment is emitted inside format_ml_table
+                if(auto k = this->format_keys(this->keys_))
+                {
+                    this->current_indent_ += fmt.name_indent;
+                    retval += this->format_comments(com, fmt.indent_type);
+                    retval += this->format_indent(fmt.indent_type);
+                    this->current_indent_ -= fmt.name_indent;
+                    retval += char_type('[');
+                    retval += k.value();
+                    retval += string_conv<string_type>("]\n");
+                }
+                // otherwise, its the root.
+
+                retval += this->format_ml_table(t, fmt);
+                return retval;
+            }
+            else if(fmt.fmt == table_format::oneline)
+            {
+                return this->format_inline_table(t, fmt);
+            }
+            else if(fmt.fmt == table_format::multiline_oneline)
+            {
+                return this->format_ml_inline_table(t, fmt);
+            }
+            else if(fmt.fmt == table_format::dotted)
+            {
+                std::vector<string_type> keys;
+                if(this->keys_.empty())
+                {
+                    throw serialization_error(format_error("toml::serializer: "
+                        "dotted table must have its key. use format(key, v)",
+                        loc, "here"), loc);
+                }
+                keys.push_back(this->keys_.back());
+
+                const auto retval = this->format_dotted_table(t, fmt, loc, keys);
+                keys.pop_back();
+                return retval;
+            }
+            else
+            {
+                assert(fmt.fmt == table_format::implicit);
+
+                string_type retval;
+                for(const auto& kv : t)
+                {
+                    const auto& k = kv.first;
+                    const auto& v = kv.second;
+
+                    if( ! v.is_table() && ! v.is_array_of_tables())
+                    {
+                        throw serialization_error(format_error("toml::serializer: "
+                            "an implicit table cannot have non-table value.",
+                            v.location(), "here"), v.location());
+                    }
+                    if(v.is_table())
+                    {
+                        if(v.as_table_fmt().fmt != table_format::multiline &&
+                           v.as_table_fmt().fmt != table_format::implicit)
+                        {
+                            throw serialization_error(format_error("toml::serializer: "
+                                "an implicit table cannot have non-multiline table",
+                                v.location(), "here"), v.location());
+                        }
+                    }
+                    else
+                    {
+                        assert(v.is_array());
+                        for(const auto& e : v.as_array())
+                        {
+                            if(e.as_table_fmt().fmt != table_format::multiline &&
+                               v.as_table_fmt().fmt != table_format::implicit)
+                            {
+                                throw serialization_error(format_error("toml::serializer: "
+                                    "an implicit table cannot have non-multiline table",
+                                    e.location(), "here"), e.location());
+                            }
+                        }
+                    }
+
+                    keys_.push_back(k);
+                    retval += (*this)(v);
+                    keys_.pop_back();
+                }
+                return retval;
+            }
+        }
+    } // }}}
+
+  private:
+
+    string_type escape_basic_string(const string_type& s) const // {{{
+    {
+        string_type retval;
+        for(const char_type c : s)
+        {
+            switch(c)
+            {
+                case char_type('\\'): {retval += string_conv<string_type>("\\\\"); break;}
+                case char_type('\"'): {retval += string_conv<string_type>("\\\""); break;}
+                case char_type('\b'): {retval += string_conv<string_type>("\\b" ); break;}
+                case char_type('\t'): {retval += string_conv<string_type>("\\t" ); break;}
+                case char_type('\f'): {retval += string_conv<string_type>("\\f" ); break;}
+                case char_type('\n'): {retval += string_conv<string_type>("\\n" ); break;}
+                case char_type('\r'): {retval += string_conv<string_type>("\\r" ); break;}
+                default  :
+                {
+                    if(c == char_type(0x1B) && spec_.v1_1_0_add_escape_sequence_e)
+                    {
+                        retval += string_conv<string_type>("\\e");
+                    }
+                    else if((char_type(0x00) <= c && c <= char_type(0x08)) ||
+                            (char_type(0x0A) <= c && c <= char_type(0x1F)) ||
+                             c == char_type(0x7F))
+                    {
+                        if(spec_.v1_1_0_add_escape_sequence_x)
+                        {
+                            retval += string_conv<string_type>("\\x");
+                        }
+                        else
+                        {
+                            retval += string_conv<string_type>("\\u00");
+                        }
+                        const auto c1 = c / 16;
+                        const auto c2 = c % 16;
+                        retval += static_cast<char_type>('0' + c1);
+                        if(c2 < 10)
+                        {
+                            retval += static_cast<char_type>('0' + c2);
+                        }
+                        else // 10 <= c2
+                        {
+                            retval += static_cast<char_type>('A' + (c2 - 10));
+                        }
+                    }
+                    else
+                    {
+                        retval += c;
+                    }
+                }
+            }
+        }
+        return retval;
+    } // }}}
+
+    string_type escape_ml_basic_string(const string_type& s) // {{{
+    {
+        string_type retval;
+        for(const char_type c : s)
+        {
+            switch(c)
+            {
+                case char_type('\\'): {retval += string_conv<string_type>("\\\\"); break;}
+                case char_type('\b'): {retval += string_conv<string_type>("\\b" ); break;}
+                case char_type('\t'): {retval += string_conv<string_type>("\\t" ); break;}
+                case char_type('\f'): {retval += string_conv<string_type>("\\f" ); break;}
+                case char_type('\n'): {retval += string_conv<string_type>("\n"  ); break;}
+                case char_type('\r'): {retval += string_conv<string_type>("\\r" ); break;}
+                default  :
+                {
+                    if(c == char_type(0x1B) && spec_.v1_1_0_add_escape_sequence_e)
+                    {
+                        retval += string_conv<string_type>("\\e");
+                    }
+                    else if((char_type(0x00) <= c && c <= char_type(0x08)) ||
+                            (char_type(0x0A) <= c && c <= char_type(0x1F)) ||
+                            c == char_type(0x7F))
+                    {
+                        if(spec_.v1_1_0_add_escape_sequence_x)
+                        {
+                            retval += string_conv<string_type>("\\x");
+                        }
+                        else
+                        {
+                            retval += string_conv<string_type>("\\u00");
+                        }
+                        const auto c1 = c / 16;
+                        const auto c2 = c % 16;
+                        retval += static_cast<char_type>('0' + c1);
+                        if(c2 < 10)
+                        {
+                            retval += static_cast<char_type>('0' + c2);
+                        }
+                        else // 10 <= c2
+                        {
+                            retval += static_cast<char_type>('A' + (c2 - 10));
+                        }
+                    }
+                    else
+                    {
+                        retval += c;
+                    }
+                }
+            }
+        }
+        // Only 1 or 2 consecutive `"`s are allowed in multiline basic string.
+        // 3 consecutive `"`s are considered as a closing delimiter.
+        // We need to check if there are 3 or more consecutive `"`s and insert
+        // backslash to break them down into several short `"`s like the `str6`
+        // in the following example.
+        // ```toml
+        // str4 = """Here are two quotation marks: "". Simple enough."""
+        // # str5 = """Here are three quotation marks: """."""  # INVALID
+        // str5 = """Here are three quotation marks: ""\"."""
+        // str6 = """Here are fifteen quotation marks: ""\"""\"""\"""\"""\"."""
+        // ```
+        auto found_3_quotes = retval.find(string_conv<string_type>("\"\"\""));
+        while(found_3_quotes != string_type::npos)
+        {
+            retval.replace(found_3_quotes, 3, string_conv<string_type>("\"\"\\\""));
+            found_3_quotes = retval.find(string_conv<string_type>("\"\"\""));
+        }
+        return retval;
+    } // }}}
+
+    string_type format_local_time(const local_time_type& t, const bool has_seconds, const std::size_t subsec_prec) // {{{
+    {
+        std::ostringstream oss;
+        oss << std::setfill('0') << std::setw(2) << static_cast<int>(t.hour);
+        oss << ':';
+        oss << std::setfill('0') << std::setw(2) << static_cast<int>(t.minute);
+        if(has_seconds)
+        {
+            oss << ':';
+            oss << std::setfill('0') << std::setw(2) << static_cast<int>(t.second);
+            if(subsec_prec != 0)
+            {
+                std::ostringstream subsec;
+                subsec << std::setfill('0') << std::setw(3) << static_cast<int>(t.millisecond);
+                subsec << std::setfill('0') << std::setw(3) << static_cast<int>(t.microsecond);
+                subsec << std::setfill('0') << std::setw(3) << static_cast<int>(t.nanosecond);
+                std::string subsec_str = subsec.str();
+                oss << '.' << subsec_str.substr(0, subsec_prec);
+            }
+        }
+        return string_conv<string_type>(oss.str());
+    } // }}}
+
+    string_type format_ml_table(const table_type& t, const table_format_info& fmt) // {{{
+    {
+        const auto format_later = [](const value_type& v) -> bool {
+
+            const bool is_ml_table = v.is_table() &&
+                v.as_table_fmt().fmt != table_format::oneline           &&
+                v.as_table_fmt().fmt != table_format::multiline_oneline &&
+                v.as_table_fmt().fmt != table_format::dotted ;
+
+            const bool is_ml_array_table = v.is_array_of_tables() &&
+                v.as_array_fmt().fmt != array_format::oneline &&
+                v.as_array_fmt().fmt != array_format::multiline;
+
+            return is_ml_table || is_ml_array_table;
+        };
+
+        string_type retval;
+        this->current_indent_ += fmt.body_indent;
+        for(const auto& kv : t)
+        {
+            const auto& key = kv.first;
+            const auto& val = kv.second;
+            if(format_later(val))
+            {
+                continue;
+            }
+            this->keys_.push_back(key);
+
+            retval += format_comments(val.comments(), fmt.indent_type);
+            retval += format_indent(fmt.indent_type);
+            if(val.is_table() && val.as_table_fmt().fmt == table_format::dotted)
+            {
+                retval += (*this)(val);
+            }
+            else
+            {
+                retval += format_key(key);
+                retval += string_conv<string_type>(" = ");
+                retval += (*this)(val);
+                retval += char_type('\n');
+            }
+            this->keys_.pop_back();
+        }
+        this->current_indent_ -= fmt.body_indent;
+
+        if( ! retval.empty())
+        {
+            retval += char_type('\n'); // for readability, add empty line between tables
+        }
+        for(const auto& kv : t)
+        {
+            if( ! format_later(kv.second))
+            {
+                continue;
+            }
+            // must be a [multiline.table] or [[multiline.array.of.tables]].
+            // comments will be generated inside it.
+            this->keys_.push_back(kv.first);
+            retval += (*this)(kv.second);
+            this->keys_.pop_back();
+        }
+        return retval;
+    } // }}}
+
+    string_type format_inline_table(const table_type& t, const table_format_info&) // {{{
+    {
+        // comments are ignored because we cannot write without newline
+        string_type retval;
+        retval += char_type('{');
+        for(const auto& kv : t)
+        {
+            this->force_inline_ = true;
+            retval += this->format_key(kv.first);
+            retval += string_conv<string_type>(" = ");
+            retval += (*this)(kv.second);
+            retval += string_conv<string_type>(", ");
+        }
+        if( ! t.empty())
+        {
+            retval.pop_back(); // ' '
+            retval.pop_back(); // ','
+        }
+        retval += char_type('}');
+        this->force_inline_ = false;
+        return retval;
+    } // }}}
+
+    string_type format_ml_inline_table(const table_type& t, const table_format_info& fmt) // {{{
+    {
+        string_type retval;
+        retval += string_conv<string_type>("{\n");
+        this->current_indent_ += fmt.body_indent;
+        for(const auto& kv : t)
+        {
+            this->force_inline_ = true;
+            retval += format_comments(kv.second.comments(), fmt.indent_type);
+            retval += format_indent(fmt.indent_type);
+            retval += kv.first;
+            retval += string_conv<string_type>(" = ");
+
+            this->force_inline_ = true;
+            retval += (*this)(kv.second);
+
+            retval += string_conv<string_type>(",\n");
+        }
+        if( ! t.empty())
+        {
+            retval.pop_back(); // '\n'
+            retval.pop_back(); // ','
+        }
+        this->current_indent_ -= fmt.body_indent;
+        this->force_inline_ = false;
+
+        this->current_indent_ += fmt.closing_indent;
+        retval += format_indent(fmt.indent_type);
+        this->current_indent_ -= fmt.closing_indent;
+
+        retval += char_type('}');
+        return retval;
+    } // }}}
+
+    string_type format_dotted_table(const table_type& t, const table_format_info& fmt, // {{{
+            const source_location&, std::vector<string_type>& keys)
+    {
+        // lets say we have: `{"a": {"b": {"c": {"d": "foo", "e": "bar"} } }`
+        // and `a` and `b` are `dotted`.
+        //
+        // - in case if `c` is `oneline`:
+        // ```toml
+        // a.b.c = {d = "foo", e = "bar"}
+        // ```
+        //
+        // - in case if and `c` is `dotted`:
+        // ```toml
+        // a.b.c.d = "foo"
+        // a.b.c.e = "bar"
+        // ```
+
+        string_type retval;
+
+        for(const auto& kv : t)
+        {
+            const auto& key = kv.first;
+            const auto& val = kv.second;
+
+            keys.push_back(key);
+
+            // format recursive dotted table?
+            if (val.is_table() &&
+                val.as_table_fmt().fmt != table_format::oneline &&
+                val.as_table_fmt().fmt != table_format::multiline_oneline)
+            {
+                retval += this->format_dotted_table(val.as_table(), val.as_table_fmt(), val.location(), keys);
+            }
+            else // non-table or inline tables. format normally
+            {
+                retval += format_comments(val.comments(), fmt.indent_type);
+                retval += format_indent(fmt.indent_type);
+                retval += format_keys(keys).value();
+                retval += string_conv<string_type>(" = ");
+                this->force_inline_ = true; // sub-table must be inlined
+                retval += (*this)(val);
+                retval += char_type('\n');
+                this->force_inline_ = false;
+            }
+            keys.pop_back();
+        }
+        return retval;
+    } // }}}
+
+    string_type format_key(const key_type& key) // {{{
+    {
+        if(key.empty())
+        {
+            return string_conv<string_type>("\"\"");
+        }
+
+        // check the key can be a bare (unquoted) key
+        auto loc = detail::make_temporary_location(string_conv<std::string>(key));
+        auto reg = detail::syntax::unquoted_key(this->spec_).scan(loc);
+        if(reg.is_ok() && loc.eof())
+        {
+            return key;
+        }
+
+        //if it includes special characters, then format it in a "quoted" key.
+        string_type formatted = string_conv<string_type>("\"");
+        for(const char_type c : key)
+        {
+            switch(c)
+            {
+                case char_type('\\'): {formatted += string_conv<string_type>("\\\\"); break;}
+                case char_type('\"'): {formatted += string_conv<string_type>("\\\""); break;}
+                case char_type('\b'): {formatted += string_conv<string_type>("\\b" ); break;}
+                case char_type('\t'): {formatted += string_conv<string_type>("\\t" ); break;}
+                case char_type('\f'): {formatted += string_conv<string_type>("\\f" ); break;}
+                case char_type('\n'): {formatted += string_conv<string_type>("\\n" ); break;}
+                case char_type('\r'): {formatted += string_conv<string_type>("\\r" ); break;}
+                default  :
+                {
+                    // ASCII ctrl char
+                    if( (char_type(0x00) <= c && c <= char_type(0x08)) ||
+                        (char_type(0x0A) <= c && c <= char_type(0x1F)) ||
+                        c == char_type(0x7F))
+                    {
+                        if(spec_.v1_1_0_add_escape_sequence_x)
+                        {
+                            formatted += string_conv<string_type>("\\x");
+                        }
+                        else
+                        {
+                            formatted += string_conv<string_type>("\\u00");
+                        }
+                        const auto c1 = c / 16;
+                        const auto c2 = c % 16;
+                        formatted += static_cast<char_type>('0' + c1);
+                        if(c2 < 10)
+                        {
+                            formatted += static_cast<char_type>('0' + c2);
+                        }
+                        else // 10 <= c2
+                        {
+                            formatted += static_cast<char_type>('A' + (c2 - 10));
+                        }
+                    }
+                    else
+                    {
+                        formatted += c;
+                    }
+                    break;
+                }
+            }
+        }
+        formatted += string_conv<string_type>("\"");
+        return formatted;
+    } // }}}
+    cxx::optional<string_type> format_keys(const std::vector<key_type>& keys) // {{{
+    {
+        if(keys.empty())
+        {
+            return cxx::make_nullopt();
+        }
+
+        string_type formatted;
+        for(const auto& ky : keys)
+        {
+            formatted += format_key(ky);
+            formatted += char_type('.');
+        }
+        formatted.pop_back(); // remove the last dot '.'
+        return formatted;
+    } // }}}
+
+    string_type format_comments(const discard_comments&, const indent_char) const // {{{
+    {
+        return string_conv<string_type>("");
+    } // }}}
+    string_type format_comments(const preserve_comments& comments, const indent_char indent_type) const // {{{
+    {
+        string_type retval;
+        for(const auto& c : comments)
+        {
+            if(c.empty()) {continue;}
+            retval += format_indent(indent_type);
+            if(c.front() != '#') {retval += char_type('#');}
+            retval += string_conv<string_type>(c);
+            if(c.back() != '\n') {retval += char_type('\n');}
+        }
+        return retval;
+    } // }}}
+
+    string_type format_indent(const indent_char indent_type) const // {{{
+    {
+        const auto indent = static_cast<std::size_t>((std::max)(0, this->current_indent_));
+        if(indent_type == indent_char::space)
+        {
+            return string_conv<string_type>(make_string(indent, ' '));
+        }
+        else if(indent_type == indent_char::tab)
+        {
+            return string_conv<string_type>(make_string(indent, '\t'));
+        }
+        else
+        {
+            return string_type{};
+        }
+    } // }}}
+
+    std::locale set_locale(std::ostream& os) const
+    {
+        return os.imbue(std::locale::classic());
+    }
+
+  private:
+
+    spec spec_;
+    bool force_inline_; // table inside an array without fmt specification
+    std::int32_t current_indent_;
+    std::vector<key_type> keys_;
+};
+} // detail
+
+template<typename TC>
+typename basic_value<TC>::string_type
+format(const basic_value<TC>& v, const spec s = spec::default_version())
+{
+    detail::serializer<TC> ser(s);
+    return ser(v);
+}
+template<typename TC>
+typename basic_value<TC>::string_type
+format(const typename basic_value<TC>::key_type& k,
+       const basic_value<TC>& v,
+       const spec s = spec::default_version())
+{
+    detail::serializer<TC> ser(s);
+    return ser(k, v);
+}
+template<typename TC>
+typename basic_value<TC>::string_type
+format(const std::vector<typename basic_value<TC>::key_type>& ks,
+       const basic_value<TC>& v,
+       const spec s = spec::default_version())
+{
+    detail::serializer<TC> ser(s);
+    return ser(ks, v);
+}
+
+template<typename TC>
+std::ostream& operator<<(std::ostream& os, const basic_value<TC>& v)
+{
+    os << format(v);
+    return os;
+}
+
+} // toml
+
+#if defined(TOML11_COMPILE_SOURCES)
+namespace toml
+{
+struct type_config;
+struct ordered_type_config;
+
+extern template typename basic_value<type_config>::string_type
+format<type_config>(const basic_value<type_config>&, const spec);
+
+extern template typename basic_value<type_config>::string_type
+format<type_config>(const typename basic_value<type_config>::key_type& k,
+                    const basic_value<type_config>& v, const spec);
+
+extern template typename basic_value<type_config>::string_type
+format<type_config>(const std::vector<typename basic_value<type_config>::key_type>& ks,
+                    const basic_value<type_config>& v, const spec s);
+
+extern template typename basic_value<type_config>::string_type
+format<ordered_type_config>(const basic_value<ordered_type_config>&, const spec);
+
+extern template typename basic_value<type_config>::string_type
+format<ordered_type_config>(const typename basic_value<ordered_type_config>::key_type& k,
+                            const basic_value<ordered_type_config>& v, const spec);
+
+extern template typename basic_value<type_config>::string_type
+format<ordered_type_config>(const std::vector<typename basic_value<ordered_type_config>::key_type>& ks,
+                            const basic_value<ordered_type_config>& v, const spec s);
+
+namespace detail
+{
+extern template class serializer<::toml::type_config>;
+extern template class serializer<::toml::ordered_type_config>;
+} // detail
+} // toml
+#endif // TOML11_COMPILE_SOURCES
+
+
+#endif // TOML11_SERIALIZER_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/skip.hpp b/src/frontend/qt_sdl/toml/toml11/skip.hpp
new file mode 100644
index 00000000..5a3b1b79
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/skip.hpp
@@ -0,0 +1,392 @@
+#ifndef TOML11_SKIP_HPP
+#define TOML11_SKIP_HPP
+
+#include "context.hpp"
+#include "region.hpp"
+#include "scanner.hpp"
+#include "syntax.hpp"
+#include "types.hpp"
+
+#include <cassert>
+
+namespace toml
+{
+namespace detail
+{
+
+template<typename TC>
+bool skip_whitespace(location& loc, const context<TC>& ctx)
+{
+    return syntax::ws(ctx.toml_spec()).scan(loc).is_ok();
+}
+
+template<typename TC>
+bool skip_empty_lines(location& loc, const context<TC>& ctx)
+{
+    return repeat_at_least(1, sequence(
+            syntax::ws(ctx.toml_spec()),
+            syntax::newline(ctx.toml_spec())
+        )).scan(loc).is_ok();
+}
+
+// For error recovery.
+//
+// In case if a comment line contains an invalid character, we need to skip it
+// to advance parsing.
+template<typename TC>
+void skip_comment_block(location& loc, const context<TC>& ctx)
+{
+    while( ! loc.eof())
+    {
+        skip_whitespace(loc, ctx);
+        if(loc.current() == '#')
+        {
+            while( ! loc.eof())
+            {
+                // both CRLF and LF ends with LF.
+                if(loc.current() == '\n')
+                {
+                    loc.advance();
+                    break;
+                }
+            }
+        }
+        else if(syntax::newline(ctx.toml_spec()).scan(loc).is_ok())
+        {
+            ; // an empty line. skip this also
+        }
+        else
+        {
+            // the next token is neither a comment nor empty line.
+            return ;
+        }
+    }
+    return ;
+}
+
+template<typename TC>
+void skip_empty_or_comment_lines(location& loc, const context<TC>& ctx)
+{
+    const auto& spec = ctx.toml_spec();
+    repeat_at_least(0, sequence(
+            syntax::ws(spec),
+            maybe(syntax::comment(spec)),
+            syntax::newline(spec))
+        ).scan(loc);
+    return ;
+}
+
+// For error recovery.
+//
+// Sometimes we need to skip a value and find a delimiter, like `,`, `]`, or `}`.
+// To find delimiter, we need to skip delimiters in a string.
+// Since we are skipping invalid value while error recovery, we don't need
+// to check the syntax. Here we just skip string-like region until closing quote
+// is found.
+template<typename TC>
+void skip_string_like(location& loc, const context<TC>&)
+{
+    // if """ is found, skip until the closing """ is found.
+    if(literal("\"\"\"").scan(loc).is_ok())
+    {
+        while( ! loc.eof())
+        {
+            if(literal("\"\"\"").scan(loc).is_ok())
+            {
+                return;
+            }
+            loc.advance();
+        }
+    }
+    else if(literal("'''").scan(loc).is_ok())
+    {
+        while( ! loc.eof())
+        {
+            if(literal("'''").scan(loc).is_ok())
+            {
+                return;
+            }
+            loc.advance();
+        }
+    }
+    // if " is found, skip until the closing " or newline is found.
+    else if(loc.current() == '"')
+    {
+        while( ! loc.eof())
+        {
+            loc.advance();
+            if(loc.current() == '"' || loc.current() == '\n')
+            {
+                loc.advance();
+                return;
+            }
+        }
+    }
+    else if(loc.current() == '\'')
+    {
+        while( ! loc.eof())
+        {
+            loc.advance();
+            if(loc.current() == '\'' || loc.current() == '\n')
+            {
+                loc.advance();
+                return ;
+            }
+        }
+    }
+    return;
+}
+
+template<typename TC>
+void skip_value(location& loc, const context<TC>& ctx);
+template<typename TC>
+void skip_array_like(location& loc, const context<TC>& ctx);
+template<typename TC>
+void skip_inline_table_like(location& loc, const context<TC>& ctx);
+template<typename TC>
+void skip_key_value_pair(location& loc, const context<TC>& ctx);
+
+template<typename TC>
+result<value_t, error_info>
+guess_value_type(const location& loc, const context<TC>& ctx);
+
+template<typename TC>
+void skip_array_like(location& loc, const context<TC>& ctx)
+{
+    const auto& spec = ctx.toml_spec();
+    assert(loc.current() == '[');
+    loc.advance();
+
+    while( ! loc.eof())
+    {
+        if(loc.current() == '\"' || loc.current() == '\'')
+        {
+            skip_string_like(loc, ctx);
+        }
+        else if(loc.current() == '#')
+        {
+            skip_comment_block(loc, ctx);
+        }
+        else if(loc.current() == '{')
+        {
+            skip_inline_table_like(loc, ctx);
+        }
+        else if(loc.current() == '[')
+        {
+            const auto checkpoint = loc;
+            if(syntax::std_table(spec).scan(loc).is_ok() ||
+               syntax::array_table(spec).scan(loc).is_ok())
+            {
+                loc = checkpoint;
+                break;
+            }
+            // if it is not a table-definition, then it is an array.
+            skip_array_like(loc, ctx);
+        }
+        else if(loc.current() == '=')
+        {
+            // key-value pair cannot be inside the array.
+            // guessing the error is "missing closing bracket `]`".
+            // find the previous key just before `=`.
+            while(loc.get_location() != 0)
+            {
+                loc.retrace();
+                if(loc.current() == '\n')
+                {
+                    loc.advance();
+                    break;
+                }
+            }
+            break;
+        }
+        else if(loc.current() == ']')
+        {
+            break; // found closing bracket
+        }
+        else
+        {
+            loc.advance();
+        }
+    }
+    return ;
+}
+
+template<typename TC>
+void skip_inline_table_like(location& loc, const context<TC>& ctx)
+{
+    assert(loc.current() == '{');
+    loc.advance();
+
+    const auto& spec = ctx.toml_spec();
+
+    while( ! loc.eof())
+    {
+        if(loc.current() == '\n' && ! spec.v1_1_0_allow_newlines_in_inline_tables)
+        {
+            break; // missing closing `}`.
+        }
+        else if(loc.current() == '\"' || loc.current() == '\'')
+        {
+            skip_string_like(loc, ctx);
+        }
+        else if(loc.current() == '#')
+        {
+            skip_comment_block(loc, ctx);
+            if( ! spec.v1_1_0_allow_newlines_in_inline_tables)
+            {
+                // comment must end with newline.
+                break; // missing closing `}`.
+            }
+        }
+        else if(loc.current() == '[')
+        {
+            const auto checkpoint = loc;
+            if(syntax::std_table(spec).scan(loc).is_ok() ||
+               syntax::array_table(spec).scan(loc).is_ok())
+            {
+                loc = checkpoint;
+                break; // missing closing `}`.
+            }
+            // if it is not a table-definition, then it is an array.
+            skip_array_like(loc, ctx);
+        }
+        else if(loc.current() == '{')
+        {
+            skip_inline_table_like(loc, ctx);
+        }
+        else if(loc.current() == '}')
+        {
+            // closing brace found. guessing the error is inside the table.
+            break;
+        }
+        else
+        {
+            // skip otherwise.
+            loc.advance();
+        }
+    }
+    return ;
+}
+
+template<typename TC>
+void skip_value(location& loc, const context<TC>& ctx)
+{
+    value_t ty = guess_value_type(loc, ctx).unwrap_or(value_t::empty);
+    if(ty == value_t::string)
+    {
+        skip_string_like(loc, ctx);
+    }
+    else if(ty == value_t::array)
+    {
+        skip_array_like(loc, ctx);
+    }
+    else if(ty == value_t::table)
+    {
+        // In case of multiline tables, it may skip key-value pair but not the
+        // whole table.
+        skip_inline_table_like(loc, ctx);
+    }
+    else // others are an "in-line" values. skip until the next line
+    {
+        while( ! loc.eof())
+        {
+            if(loc.current() == '\n')
+            {
+                break;
+            }
+            else if(loc.current() == ',' || loc.current() == ']' || loc.current() == '}')
+            {
+                break;
+            }
+            loc.advance();
+        }
+    }
+    return;
+}
+
+template<typename TC>
+void skip_key_value_pair(location& loc, const context<TC>& ctx)
+{
+    while( ! loc.eof())
+    {
+        if(loc.current() == '=')
+        {
+            skip_whitespace(loc, ctx);
+            skip_value(loc, ctx);
+            return;
+        }
+        else if(loc.current() == '\n')
+        {
+            // newline is found before finding `=`. assuming "missing `=`".
+            return;
+        }
+        loc.advance();
+    }
+    return ;
+}
+
+template<typename TC>
+void skip_until_next_table(location& loc, const context<TC>& ctx)
+{
+    const auto& spec = ctx.toml_spec();
+    while( ! loc.eof())
+    {
+        if(loc.current() == '\n')
+        {
+            loc.advance();
+            const auto line_begin = loc;
+
+            skip_whitespace(loc, ctx);
+            if(syntax::std_table(spec).scan(loc).is_ok())
+            {
+                loc = line_begin;
+                return ;
+            }
+            if(syntax::array_table(spec).scan(loc).is_ok())
+            {
+                loc = line_begin;
+                return ;
+            }
+        }
+        loc.advance();
+    }
+}
+
+} // namespace detail
+} // namespace toml
+
+#if defined(TOML11_COMPILE_SOURCES)
+namespace toml
+{
+struct type_config;
+struct ordered_type_config;
+
+namespace detail
+{
+extern template bool skip_whitespace            <type_config>(location& loc, const context<type_config>&);
+extern template bool skip_empty_lines           <type_config>(location& loc, const context<type_config>&);
+extern template void skip_comment_block         <type_config>(location& loc, const context<type_config>&);
+extern template void skip_empty_or_comment_lines<type_config>(location& loc, const context<type_config>&);
+extern template void skip_string_like           <type_config>(location& loc, const context<type_config>&);
+extern template void skip_array_like            <type_config>(location& loc, const context<type_config>&);
+extern template void skip_inline_table_like     <type_config>(location& loc, const context<type_config>&);
+extern template void skip_value                 <type_config>(location& loc, const context<type_config>&);
+extern template void skip_key_value_pair        <type_config>(location& loc, const context<type_config>&);
+extern template void skip_until_next_table      <type_config>(location& loc, const context<type_config>&);
+
+extern template bool skip_whitespace            <ordered_type_config>(location& loc, const context<ordered_type_config>&);
+extern template bool skip_empty_lines           <ordered_type_config>(location& loc, const context<ordered_type_config>&);
+extern template void skip_comment_block         <ordered_type_config>(location& loc, const context<ordered_type_config>&);
+extern template void skip_empty_or_comment_lines<ordered_type_config>(location& loc, const context<ordered_type_config>&);
+extern template void skip_string_like           <ordered_type_config>(location& loc, const context<ordered_type_config>&);
+extern template void skip_array_like            <ordered_type_config>(location& loc, const context<ordered_type_config>&);
+extern template void skip_inline_table_like     <ordered_type_config>(location& loc, const context<ordered_type_config>&);
+extern template void skip_value                 <ordered_type_config>(location& loc, const context<ordered_type_config>&);
+extern template void skip_key_value_pair        <ordered_type_config>(location& loc, const context<ordered_type_config>&);
+extern template void skip_until_next_table      <ordered_type_config>(location& loc, const context<ordered_type_config>&);
+
+} // detail
+} // toml
+#endif // TOML11_COMPILE_SOURCES
+
+#endif // TOML11_SKIP_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/source_location.hpp b/src/frontend/qt_sdl/toml/toml11/source_location.hpp
new file mode 100644
index 00000000..02e62851
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/source_location.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_SOURCE_LOCATION_HPP
+#define TOML11_SOURCE_LOCATION_HPP
+
+#include "fwd/source_location_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/source_location_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_SOURCE_LOCATION_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/spec.hpp b/src/frontend/qt_sdl/toml/toml11/spec.hpp
new file mode 100644
index 00000000..ca0a9962
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/spec.hpp
@@ -0,0 +1,121 @@
+#ifndef TOML11_SPEC_HPP
+#define TOML11_SPEC_HPP
+
+#include <ostream>
+#include <sstream>
+
+#include <cstdint>
+
+namespace toml
+{
+
+struct semantic_version
+{
+    constexpr semantic_version(std::uint32_t mjr, std::uint32_t mnr, std::uint32_t p) noexcept
+        : major{mjr}, minor{mnr}, patch{p}
+    {}
+
+    std::uint32_t major;
+    std::uint32_t minor;
+    std::uint32_t patch;
+};
+
+constexpr inline semantic_version
+make_semver(std::uint32_t mjr, std::uint32_t mnr, std::uint32_t p) noexcept
+{
+    return semantic_version(mjr, mnr, p);
+}
+
+constexpr inline bool
+operator==(const semantic_version& lhs, const semantic_version& rhs) noexcept
+{
+    return lhs.major == rhs.major &&
+           lhs.minor == rhs.minor &&
+           lhs.patch == rhs.patch;
+}
+constexpr inline bool
+operator!=(const semantic_version& lhs, const semantic_version& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+constexpr inline bool
+operator<(const semantic_version& lhs, const semantic_version& rhs) noexcept
+{
+    return lhs.major < rhs.major ||
+           (lhs.major == rhs.major && lhs.minor < rhs.minor) ||
+           (lhs.major == rhs.major && lhs.minor == rhs.minor && lhs.patch < rhs.patch);
+}
+constexpr inline bool
+operator>(const semantic_version& lhs, const semantic_version& rhs) noexcept
+{
+    return rhs < lhs;
+}
+constexpr inline bool
+operator<=(const semantic_version& lhs, const semantic_version& rhs) noexcept
+{
+    return !(lhs > rhs);
+}
+constexpr inline bool
+operator>=(const semantic_version& lhs, const semantic_version& rhs) noexcept
+{
+    return !(lhs < rhs);
+}
+
+inline std::ostream& operator<<(std::ostream& os, const semantic_version& v)
+{
+    os << v.major << '.' << v.minor << '.' << v.patch;
+    return os;
+}
+
+inline std::string to_string(const semantic_version& v)
+{
+    std::ostringstream oss;
+    oss << v;
+    return oss.str();
+}
+
+struct spec
+{
+    constexpr static spec default_version() noexcept
+    {
+        return spec::v(1, 0, 0);
+    }
+
+    constexpr static spec v(std::uint32_t mjr, std::uint32_t mnr, std::uint32_t p) noexcept
+    {
+        return spec(make_semver(mjr, mnr, p));
+    }
+
+    constexpr explicit spec(const semantic_version& semver) noexcept
+        : version{semver},
+          v1_1_0_allow_control_characters_in_comments {semantic_version{1, 1, 0} <= semver},
+          v1_1_0_allow_newlines_in_inline_tables      {semantic_version{1, 1, 0} <= semver},
+          v1_1_0_allow_trailing_comma_in_inline_tables{semantic_version{1, 1, 0} <= semver},
+          v1_1_0_allow_non_english_in_bare_keys       {semantic_version{1, 1, 0} <= semver},
+          v1_1_0_add_escape_sequence_e                {semantic_version{1, 1, 0} <= semver},
+          v1_1_0_add_escape_sequence_x                {semantic_version{1, 1, 0} <= semver},
+          v1_1_0_make_seconds_optional                {semantic_version{1, 1, 0} <= semver},
+          ext_hex_float {false},
+          ext_num_suffix{false},
+          ext_null_value{false}
+    {}
+
+    semantic_version version; // toml version
+
+    // diff from v1.0.0 -> v1.1.0
+    bool v1_1_0_allow_control_characters_in_comments;
+    bool v1_1_0_allow_newlines_in_inline_tables;
+    bool v1_1_0_allow_trailing_comma_in_inline_tables;
+    bool v1_1_0_allow_non_english_in_bare_keys;
+    bool v1_1_0_add_escape_sequence_e;
+    bool v1_1_0_add_escape_sequence_x;
+    bool v1_1_0_make_seconds_optional;
+
+    // library extensions
+    bool ext_hex_float;  // allow hex float (in C++ style)
+    bool ext_num_suffix; // allow number suffix (in C++ style)
+    bool ext_null_value; // allow `null` as a value
+};
+
+} // namespace toml
+#endif // TOML11_SPEC_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/storage.hpp b/src/frontend/qt_sdl/toml/toml11/storage.hpp
new file mode 100644
index 00000000..b63e24f6
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/storage.hpp
@@ -0,0 +1,49 @@
+#ifndef TOML11_STORAGE_HPP
+#define TOML11_STORAGE_HPP
+
+#include "compat.hpp"
+
+namespace toml
+{
+namespace detail
+{
+
+// It owns a pointer to T. It does deep-copy when copied.
+// This struct is introduced to implement a recursive type.
+//
+// `toml::value` contains `std::vector<toml::value>` to represent a toml array.
+// But, in the definition of `toml::value`, `toml::value` is still incomplete.
+// `std::vector` of an incomplete type is not allowed in C++11 (it is allowed
+// after C++17). To avoid this, we need to use a pointer to `toml::value`, like
+// `std::vector<std::unique_ptr<toml::value>>`. Although `std::unique_ptr` is
+// noncopyable, we want to make `toml::value` copyable. `storage` is introduced
+// to resolve those problems.
+template<typename T>
+struct storage
+{
+    using value_type = T;
+
+    explicit storage(value_type v): ptr_(cxx::make_unique<T>(std::move(v))) {}
+    ~storage() = default;
+
+    storage(const storage& rhs): ptr_(cxx::make_unique<T>(*rhs.ptr_)) {}
+    storage& operator=(const storage& rhs)
+    {
+        this->ptr_ = cxx::make_unique<T>(*rhs.ptr_);
+        return *this;
+    }
+
+    storage(storage&&) = default;
+    storage& operator=(storage&&) = default;
+
+    bool is_ok() const noexcept {return static_cast<bool>(ptr_);}
+
+    value_type& get() const noexcept {return *ptr_;}
+
+  private:
+    std::unique_ptr<value_type> ptr_;
+};
+
+} // detail
+} // toml
+#endif // TOML11_STORAGE_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/syntax.hpp b/src/frontend/qt_sdl/toml/toml11/syntax.hpp
new file mode 100644
index 00000000..912535de
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/syntax.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_SYNTAX_HPP
+#define TOML11_SYNTAX_HPP
+
+#include "fwd/syntax_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/syntax_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif// TOML11_SYNTAX_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/traits.hpp b/src/frontend/qt_sdl/toml/toml11/traits.hpp
new file mode 100644
index 00000000..7a8832f2
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/traits.hpp
@@ -0,0 +1,240 @@
+#ifndef TOML11_TRAITS_HPP
+#define TOML11_TRAITS_HPP
+
+#include "from.hpp"
+#include "into.hpp"
+#include "compat.hpp"
+
+#include <array>
+#include <chrono>
+#include <forward_list>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#if defined(TOML11_HAS_STRING_VIEW)
+#include <string_view>
+#endif
+
+namespace toml
+{
+template<typename TypeConcig>
+class basic_value;
+
+namespace detail
+{
+// ---------------------------------------------------------------------------
+// check whether type T is a kind of container/map class
+
+struct has_iterator_impl
+{
+    template<typename T> static std::true_type  check(typename T::iterator*);
+    template<typename T> static std::false_type check(...);
+};
+struct has_value_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::value_type*);
+    template<typename T> static std::false_type check(...);
+};
+struct has_key_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::key_type*);
+    template<typename T> static std::false_type check(...);
+};
+struct has_mapped_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::mapped_type*);
+    template<typename T> static std::false_type check(...);
+};
+struct has_reserve_method_impl
+{
+    template<typename T> static std::false_type check(...);
+    template<typename T> static std::true_type  check(
+        decltype(std::declval<T>().reserve(std::declval<std::size_t>()))*);
+};
+struct has_push_back_method_impl
+{
+    template<typename T> static std::false_type check(...);
+    template<typename T> static std::true_type  check(
+        decltype(std::declval<T>().push_back(std::declval<typename T::value_type>()))*);
+};
+struct is_comparable_impl
+{
+    template<typename T> static std::false_type check(...);
+    template<typename T> static std::true_type  check(
+        decltype(std::declval<T>() < std::declval<T>())*);
+};
+
+struct has_from_toml_method_impl
+{
+    template<typename T, typename TC>
+    static std::true_type  check(
+        decltype(std::declval<T>().from_toml(std::declval<::toml::basic_value<TC>>()))*);
+
+    template<typename T, typename TC>
+    static std::false_type check(...);
+};
+struct has_into_toml_method_impl
+{
+    template<typename T>
+    static std::true_type  check(decltype(std::declval<T>().into_toml())*);
+    template<typename T>
+    static std::false_type check(...);
+};
+
+struct has_template_into_toml_method_impl
+{
+    template<typename T, typename TypeConfig>
+    static std::true_type  check(decltype(std::declval<T>().template into_toml<TypeConfig>())*);
+    template<typename T, typename TypeConfig>
+    static std::false_type check(...);
+};
+
+struct has_specialized_from_impl
+{
+    template<typename T>
+    static std::false_type check(...);
+    template<typename T, std::size_t S = sizeof(::toml::from<T>)>
+    static std::true_type check(::toml::from<T>*);
+};
+struct has_specialized_into_impl
+{
+    template<typename T>
+    static std::false_type check(...);
+    template<typename T, std::size_t S = sizeof(::toml::into<T>)>
+    static std::true_type check(::toml::into<T>*);
+};
+
+
+/// Intel C++ compiler can not use decltype in parent class declaration, here
+/// is a hack to work around it. https://stackoverflow.com/a/23953090/4692076
+#ifdef __INTEL_COMPILER
+#define decltype(...) std::enable_if<true, decltype(__VA_ARGS__)>::type
+#endif
+
+template<typename T>
+struct has_iterator: decltype(has_iterator_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_value_type: decltype(has_value_type_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_key_type: decltype(has_key_type_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_mapped_type: decltype(has_mapped_type_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_reserve_method: decltype(has_reserve_method_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_push_back_method: decltype(has_push_back_method_impl::check<T>(nullptr)){};
+template<typename T>
+struct is_comparable: decltype(is_comparable_impl::check<T>(nullptr)){};
+
+template<typename T, typename TC>
+struct has_from_toml_method: decltype(has_from_toml_method_impl::check<T, TC>(nullptr)){};
+
+template<typename T>
+struct has_into_toml_method: decltype(has_into_toml_method_impl::check<T>(nullptr)){};
+
+template<typename T, typename TypeConfig>
+struct has_template_into_toml_method: decltype(has_template_into_toml_method_impl::check<T, TypeConfig>(nullptr)){};
+
+template<typename T>
+struct has_specialized_from: decltype(has_specialized_from_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_specialized_into: decltype(has_specialized_into_impl::check<T>(nullptr)){};
+
+#ifdef __INTEL_COMPILER
+#undef decltype
+#endif
+
+// ---------------------------------------------------------------------------
+// type checkers
+
+template<typename T> struct is_std_pair_impl : std::false_type{};
+template<typename T1, typename T2>
+struct is_std_pair_impl<std::pair<T1, T2>> : std::true_type{};
+template<typename T>
+using is_std_pair = is_std_pair_impl<cxx::remove_cvref_t<T>>;
+
+template<typename T> struct is_std_tuple_impl : std::false_type{};
+template<typename ... Ts>
+struct is_std_tuple_impl<std::tuple<Ts...>> : std::true_type{};
+template<typename T>
+using is_std_tuple = is_std_tuple_impl<cxx::remove_cvref_t<T>>;
+
+template<typename T> struct is_std_array_impl : std::false_type{};
+template<typename T, std::size_t N>
+struct is_std_array_impl<std::array<T, N>> : std::true_type{};
+template<typename T>
+using is_std_array = is_std_array_impl<cxx::remove_cvref_t<T>>;
+
+template<typename T> struct is_std_forward_list_impl : std::false_type{};
+template<typename T>
+struct is_std_forward_list_impl<std::forward_list<T>> : std::true_type{};
+template<typename T>
+using is_std_forward_list = is_std_forward_list_impl<cxx::remove_cvref_t<T>>;
+
+template<typename T> struct is_std_basic_string_impl : std::false_type{};
+template<typename C, typename T, typename A>
+struct is_std_basic_string_impl<std::basic_string<C, T, A>> : std::true_type{};
+template<typename T>
+using is_std_basic_string = is_std_basic_string_impl<cxx::remove_cvref_t<T>>;
+
+template<typename T> struct is_1byte_std_basic_string_impl : std::false_type{};
+template<typename C, typename T, typename A>
+struct is_1byte_std_basic_string_impl<std::basic_string<C, T, A>>
+    : std::integral_constant<bool, sizeof(C) == sizeof(char)> {};
+template<typename T>
+using is_1byte_std_basic_string = is_std_basic_string_impl<cxx::remove_cvref_t<T>>;
+
+#if defined(TOML11_HAS_STRING_VIEW)
+template<typename T> struct is_std_basic_string_view_impl : std::false_type{};
+template<typename C, typename T>
+struct is_std_basic_string_view_impl<std::basic_string_view<C, T>> : std::true_type{};
+template<typename T>
+using is_std_basic_string_view = is_std_basic_string_view_impl<cxx::remove_cvref_t<T>>;
+
+template<typename V, typename S>
+struct is_string_view_of : std::false_type {};
+template<typename C, typename T>
+struct is_string_view_of<std::basic_string_view<C, T>, std::basic_string<C, T>> : std::true_type {};
+#endif
+
+template<typename T> struct is_chrono_duration_impl: std::false_type{};
+template<typename Rep, typename Period>
+struct is_chrono_duration_impl<std::chrono::duration<Rep, Period>>: std::true_type{};
+template<typename T>
+using is_chrono_duration = is_chrono_duration_impl<cxx::remove_cvref_t<T>>;
+
+template<typename T>
+struct is_map_impl : cxx::conjunction< // map satisfies all the following conditions
+    has_iterator<T>,         // has T::iterator
+    has_value_type<T>,       // has T::value_type
+    has_key_type<T>,         // has T::key_type
+    has_mapped_type<T>       // has T::mapped_type
+    >{};
+template<typename T>
+using is_map = is_map_impl<cxx::remove_cvref_t<T>>;
+
+template<typename T>
+struct is_container_impl : cxx::conjunction<
+    cxx::negation<is_map<T>>,                         // not a map
+    cxx::negation<std::is_same<T, std::string>>,      // not a std::string
+#ifdef TOML11_HAS_STRING_VIEW
+    cxx::negation<std::is_same<T, std::string_view>>, // not a std::string_view
+#endif
+    has_iterator<T>,                             // has T::iterator
+    has_value_type<T>                            // has T::value_type
+    >{};
+template<typename T>
+using is_container = is_container_impl<cxx::remove_cvref_t<T>>;
+
+template<typename T>
+struct is_basic_value_impl: std::false_type{};
+template<typename TC>
+struct is_basic_value_impl<::toml::basic_value<TC>>: std::true_type{};
+template<typename T>
+using is_basic_value = is_basic_value_impl<cxx::remove_cvref_t<T>>;
+
+}// detail
+}//toml
+#endif // TOML11_TRAITS_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/types.hpp b/src/frontend/qt_sdl/toml/toml11/types.hpp
new file mode 100644
index 00000000..75f55b2f
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/types.hpp
@@ -0,0 +1,397 @@
+#ifndef TOML11_TYPES_HPP
+#define TOML11_TYPES_HPP
+
+#include "comments.hpp"
+#include "error_info.hpp"
+#include "format.hpp"
+#include "ordered_map.hpp"
+#include "value.hpp"
+
+#include <ostream>
+#include <sstream>
+#include <string>
+#include <type_traits>
+#include <unordered_map>
+#include <vector>
+
+#include <cstdint>
+#include <cstdio>
+
+namespace toml
+{
+
+// forward decl
+template<typename TypeConfig>
+class basic_value;
+
+// when you use a special integer type as toml::value::integer_type, parse must
+// be able to read it. So, type_config has static member functions that read the
+// integer_type as {dec, hex, oct, bin}-integer. But, in most cases, operator<<
+// is enough. To make config easy, we provide the default read functions.
+//
+// Before this functions is called, syntax is checked and prefix(`0x` etc) and
+// spacer(`_`) are removed.
+
+template<typename T>
+result<T, error_info>
+read_dec_int(const std::string& str, const source_location src)
+{
+    constexpr auto max_digits = std::numeric_limits<T>::digits;
+    assert( ! str.empty());
+
+    T val{0};
+    std::istringstream iss(str);
+    iss >> val;
+    if(iss.fail())
+    {
+        return err(make_error_info("toml::parse_dec_integer: "
+            "too large integer: current max digits = 2^" + std::to_string(max_digits),
+            std::move(src), "must be < 2^" + std::to_string(max_digits)));
+    }
+    return ok(val);
+}
+
+template<typename T>
+result<T, error_info>
+read_hex_int(const std::string& str, const source_location src)
+{
+    constexpr auto max_digits = std::numeric_limits<T>::digits;
+    assert( ! str.empty());
+
+    T val{0};
+    std::istringstream iss(str);
+    iss >> std::hex >> val;
+    if(iss.fail())
+    {
+        return err(make_error_info("toml::parse_hex_integer: "
+            "too large integer: current max value = 2^" + std::to_string(max_digits),
+            std::move(src), "must be < 2^" + std::to_string(max_digits)));
+    }
+    return ok(val);
+}
+
+template<typename T>
+result<T, error_info>
+read_oct_int(const std::string& str, const source_location src)
+{
+    constexpr auto max_digits = std::numeric_limits<T>::digits;
+    assert( ! str.empty());
+
+    T val{0};
+    std::istringstream iss(str);
+    iss >> std::oct >> val;
+    if(iss.fail())
+    {
+        return err(make_error_info("toml::parse_oct_integer: "
+            "too large integer: current max value = 2^" + std::to_string(max_digits),
+            std::move(src), "must be < 2^" + std::to_string(max_digits)));
+    }
+    return ok(val);
+}
+
+template<typename T>
+result<T, error_info>
+read_bin_int(const std::string& str, const source_location src)
+{
+    constexpr auto is_bounded =  std::numeric_limits<T>::is_bounded;
+    constexpr auto max_digits =  std::numeric_limits<T>::digits;
+    const auto max_value  = (std::numeric_limits<T>::max)();
+
+    T val{0};
+    T base{1};
+    for(auto i = str.rbegin(); i != str.rend(); ++i)
+    {
+        const auto c = *i;
+        if(c == '1')
+        {
+            val += base;
+            // prevent `base` from overflow
+            if(is_bounded && max_value / 2 < base && std::next(i) != str.rend())
+            {
+                base = 0;
+            }
+            else
+            {
+                base *= 2;
+            }
+        }
+        else
+        {
+            assert(c == '0');
+
+            if(is_bounded && max_value / 2 < base && std::next(i) != str.rend())
+            {
+                base = 0;
+            }
+            else
+            {
+                base *= 2;
+            }
+        }
+    }
+    if(base == 0)
+    {
+        return err(make_error_info("toml::parse_bin_integer: "
+            "too large integer: current max value = 2^" + std::to_string(max_digits),
+            std::move(src), "must be < 2^" + std::to_string(max_digits)));
+    }
+    return ok(val);
+}
+
+template<typename T>
+result<T, error_info>
+read_int(const std::string& str, const source_location src, const std::uint8_t base)
+{
+    assert(base == 10 || base == 16 || base == 8 || base == 2);
+    switch(base)
+    {
+        case  2: { return read_bin_int<T>(str, src); }
+        case  8: { return read_oct_int<T>(str, src); }
+        case 16: { return read_hex_int<T>(str, src); }
+        default:
+        {
+            assert(base == 10);
+            return read_dec_int<T>(str, src);
+        }
+    }
+}
+
+inline result<float, error_info>
+read_hex_float(const std::string& str, const source_location src, float val)
+{
+#if defined(_MSC_VER) && ! defined(__clang__)
+    const auto res = ::sscanf_s(str.c_str(), "%a", std::addressof(val));
+#else
+    const auto res = std::sscanf(str.c_str(), "%a", std::addressof(val));
+#endif
+    if(res != 1)
+    {
+        return err(make_error_info("toml::parse_floating: "
+            "failed to read hexadecimal floating point value ",
+            std::move(src), "here"));
+    }
+    return ok(val);
+}
+inline result<double, error_info>
+read_hex_float(const std::string& str, const source_location src, double val)
+{
+#if defined(_MSC_VER) && ! defined(__clang__)
+    const auto res = ::sscanf_s(str.c_str(), "%la", std::addressof(val));
+#else
+    const auto res = std::sscanf(str.c_str(), "%la", std::addressof(val));
+#endif
+    if(res != 1)
+    {
+        return err(make_error_info("toml::parse_floating: "
+            "failed to read hexadecimal floating point value ",
+            std::move(src), "here"));
+    }
+    return ok(val);
+}
+template<typename T>
+cxx::enable_if_t<cxx::conjunction<
+    cxx::negation<std::is_same<cxx::remove_cvref_t<T>, double>>,
+    cxx::negation<std::is_same<cxx::remove_cvref_t<T>, float>>
+    >::value, result<T, error_info>>
+read_hex_float(const std::string&, const source_location src, T)
+{
+    return err(make_error_info("toml::parse_floating: failed to read "
+        "floating point value because of unknown type in type_config",
+        std::move(src), "here"));
+}
+
+template<typename T>
+result<T, error_info>
+read_dec_float(const std::string& str, const source_location src)
+{
+    T val;
+    std::istringstream iss(str);
+    iss >> val;
+    if(iss.fail())
+    {
+        return err(make_error_info("toml::parse_floating: "
+            "failed to read floating point value from stream",
+            std::move(src), "here"));
+    }
+    return ok(val);
+}
+
+template<typename T>
+result<T, error_info>
+read_float(const std::string& str, const source_location src, const bool is_hex)
+{
+    if(is_hex)
+    {
+        return read_hex_float(str, src, T{});
+    }
+    else
+    {
+        return read_dec_float<T>(str, src);
+    }
+}
+
+struct type_config
+{
+    using comment_type  = preserve_comments;
+
+    using boolean_type  = bool;
+    using integer_type  = std::int64_t;
+    using floating_type = double;
+    using string_type   = std::string;
+
+    template<typename T>
+    using array_type = std::vector<T>;
+    template<typename K, typename T>
+    using table_type = std::unordered_map<K, T>;
+
+    static result<integer_type, error_info>
+    parse_int(const std::string& str, const source_location src, const std::uint8_t base)
+    {
+        return read_int<integer_type>(str, src, base);
+    }
+    static result<floating_type, error_info>
+    parse_float(const std::string& str, const source_location src, const bool is_hex)
+    {
+        return read_float<floating_type>(str, src, is_hex);
+    }
+};
+
+using value = basic_value<type_config>;
+using table = typename value::table_type;
+using array = typename value::array_type;
+
+struct ordered_type_config
+{
+    using comment_type  = preserve_comments;
+
+    using boolean_type  = bool;
+    using integer_type  = std::int64_t;
+    using floating_type = double;
+    using string_type   = std::string;
+
+    template<typename T>
+    using array_type = std::vector<T>;
+    template<typename K, typename T>
+    using table_type = ordered_map<K, T>;
+
+    static result<integer_type, error_info>
+    parse_int(const std::string& str, const source_location src, const std::uint8_t base)
+    {
+        return read_int<integer_type>(str, src, base);
+    }
+    static result<floating_type, error_info>
+    parse_float(const std::string& str, const source_location src, const bool is_hex)
+    {
+        return read_float<floating_type>(str, src, is_hex);
+    }
+};
+
+using ordered_value = basic_value<ordered_type_config>;
+using ordered_table = typename ordered_value::table_type;
+using ordered_array = typename ordered_value::array_type;
+
+// ----------------------------------------------------------------------------
+// meta functions for internal use
+
+namespace detail
+{
+
+// ----------------------------------------------------------------------------
+// check if type T has all the needed member types
+
+struct has_comment_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::comment_type*);
+    template<typename T> static std::false_type check(...);
+};
+template<typename T>
+using has_comment_type = decltype(has_comment_type_impl::check<T>(nullptr));
+
+struct has_integer_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::integer_type*);
+    template<typename T> static std::false_type check(...);
+};
+template<typename T>
+using has_integer_type = decltype(has_integer_type_impl::check<T>(nullptr));
+
+struct has_floating_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::floating_type*);
+    template<typename T> static std::false_type check(...);
+};
+template<typename T>
+using has_floating_type = decltype(has_floating_type_impl::check<T>(nullptr));
+
+struct has_string_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::string_type*);
+    template<typename T> static std::false_type check(...);
+};
+template<typename T>
+using has_string_type = decltype(has_string_type_impl::check<T>(nullptr));
+
+struct has_array_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::template array_type<int>*);
+    template<typename T> static std::false_type check(...);
+};
+template<typename T>
+using has_array_type = decltype(has_array_type_impl::check<T>(nullptr));
+
+struct has_table_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::template table_type<int, int>*);
+    template<typename T> static std::false_type check(...);
+};
+template<typename T>
+using has_table_type = decltype(has_table_type_impl::check<T>(nullptr));
+
+struct has_parse_int_impl
+{
+    template<typename T> static std::true_type  check(decltype(std::declval<T>().parse_int(
+            std::declval<const std::string&>(),
+            std::declval<const source_location>(),
+            std::declval<const std::uint8_t>()
+        ))*);
+    template<typename T> static std::false_type check(...);
+};
+template<typename T>
+using has_parse_int = decltype(has_parse_int_impl::check<T>(nullptr));
+
+struct has_parse_float_impl
+{
+    template<typename T> static std::true_type  check(decltype(std::declval<T>().parse_float(
+            std::declval<const std::string&>(),
+            std::declval<const source_location>(),
+            std::declval<const bool>()
+        ))*);
+    template<typename T> static std::false_type check(...);
+};
+template<typename T>
+using has_parse_float = decltype(has_parse_float_impl::check<T>(nullptr));
+
+template<typename T>
+using is_type_config = cxx::conjunction<
+    has_comment_type<T>,
+    has_integer_type<T>,
+    has_floating_type<T>,
+    has_string_type<T>,
+    has_array_type<T>,
+    has_table_type<T>,
+    has_parse_int<T>,
+    has_parse_float<T>
+    >;
+
+} // namespace detail
+} // namespace toml
+
+#if defined(TOML11_COMPILE_SOURCES)
+namespace toml
+{
+extern template class basic_value<type_config>;
+extern template class basic_value<ordered_type_config>;
+} // toml
+#endif // TOML11_COMPILE_SOURCES
+
+#endif // TOML11_TYPES_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/utility.hpp b/src/frontend/qt_sdl/toml/toml11/utility.hpp
new file mode 100644
index 00000000..9e30ccb7
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/utility.hpp
@@ -0,0 +1,170 @@
+#ifndef TOML11_UTILITY_HPP
+#define TOML11_UTILITY_HPP
+
+#include "result.hpp"
+#include "traits.hpp"
+
+#include <array>
+#include <sstream>
+
+#include <cassert>
+#include <cctype>
+#include <cstring>
+
+namespace toml
+{
+namespace detail
+{
+
+// to output character in an error message.
+inline std::string show_char(const int c)
+{
+    using char_type = unsigned char;
+    if(std::isgraph(c))
+    {
+        return std::string(1, static_cast<char>(c));
+    }
+    else
+    {
+        std::array<char, 5> buf;
+        buf.fill('\0');
+        const auto r = std::snprintf(buf.data(), buf.size(), "0x%02x", c & 0xFF);
+        assert(r == static_cast<int>(buf.size()) - 1);
+        (void) r; // Unused variable warning
+        auto in_hex = std::string(buf.data());
+        switch(c)
+        {
+            case char_type('\0'):   {in_hex += "(NUL)";             break;}
+            case char_type(' ') :   {in_hex += "(SPACE)";           break;}
+            case char_type('\n'):   {in_hex += "(LINE FEED)";       break;}
+            case char_type('\r'):   {in_hex += "(CARRIAGE RETURN)"; break;}
+            case char_type('\t'):   {in_hex += "(TAB)";             break;}
+            case char_type('\v'):   {in_hex += "(VERTICAL TAB)";    break;}
+            case char_type('\f'):   {in_hex += "(FORM FEED)";       break;}
+            case char_type('\x1B'): {in_hex += "(ESCAPE)";          break;}
+            default: break;
+        }
+        return in_hex;
+    }
+}
+
+// ---------------------------------------------------------------------------
+
+template<typename Container>
+void try_reserve_impl(Container& container, std::size_t N, std::true_type)
+{
+    container.reserve(N);
+    return;
+}
+template<typename Container>
+void try_reserve_impl(Container&, std::size_t, std::false_type) noexcept
+{
+    return;
+}
+
+template<typename Container>
+void try_reserve(Container& container, std::size_t N)
+{
+    try_reserve_impl(container, N, has_reserve_method<Container>{});
+    return;
+}
+
+// ---------------------------------------------------------------------------
+
+template<typename T>
+result<T, none_t> from_string(const std::string& str)
+{
+    T v;
+    std::istringstream iss(str);
+    iss >> v;
+    if(iss.fail())
+    {
+        return err();
+    }
+    return ok(v);
+}
+
+// ---------------------------------------------------------------------------
+
+// helper function to avoid std::string(0, 'c') or std::string(iter, iter)
+template<typename Iterator>
+std::string make_string(Iterator first, Iterator last)
+{
+    if(first == last) {return "";}
+    return std::string(first, last);
+}
+inline std::string make_string(std::size_t len, char c)
+{
+    if(len == 0) {return "";}
+    return std::string(len, c);
+}
+
+// ---------------------------------------------------------------------------
+
+template<typename Char,  typename Traits, typename Alloc,
+         typename Char2, typename Traits2, typename Alloc2>
+struct string_conv_impl
+{
+    static_assert(sizeof(Char)  == sizeof(char), "");
+    static_assert(sizeof(Char2) == sizeof(char), "");
+
+    static std::basic_string<Char, Traits, Alloc> invoke(std::basic_string<Char2, Traits2, Alloc2> s)
+    {
+        std::basic_string<Char, Traits, Alloc> retval;
+        std::transform(s.begin(), s.end(), std::back_inserter(retval),
+            [](const Char2 c) {return static_cast<Char>(c);});
+        return retval;
+    }
+    template<std::size_t N>
+    static std::basic_string<Char, Traits, Alloc> invoke(const Char2 (&s)[N])
+    {
+        std::basic_string<Char, Traits, Alloc> retval;
+        // "string literal" has null-char at the end. to skip it, we use prev.
+        std::transform(std::begin(s), std::prev(std::end(s)), std::back_inserter(retval),
+            [](const Char2 c) {return static_cast<Char>(c);});
+        return retval;
+    }
+};
+
+template<typename Char,  typename Traits, typename Alloc>
+struct string_conv_impl<Char, Traits, Alloc, Char, Traits, Alloc>
+{
+    static_assert(sizeof(Char) == sizeof(char), "");
+
+    static std::basic_string<Char, Traits, Alloc> invoke(std::basic_string<Char, Traits, Alloc> s)
+    {
+        return s;
+    }
+    template<std::size_t N>
+    static std::basic_string<Char, Traits, Alloc> invoke(const Char (&s)[N])
+    {
+        return std::basic_string<Char, Traits, Alloc>(s);
+    }
+};
+
+template<typename S, typename Char2, typename Traits2, typename Alloc2>
+cxx::enable_if_t<is_std_basic_string<S>::value, S>
+string_conv(std::basic_string<Char2, Traits2, Alloc2> s)
+{
+    using C = typename S::value_type;
+    using T = typename S::traits_type;
+    using A = typename S::allocator_type;
+    return string_conv_impl<C, T, A, Char2, Traits2, Alloc2>::invoke(std::move(s));
+}
+template<typename S, std::size_t N>
+cxx::enable_if_t<is_std_basic_string<S>::value, S>
+string_conv(const char (&s)[N])
+{
+    using C = typename S::value_type;
+    using T = typename S::traits_type;
+    using A = typename S::allocator_type;
+    using C2 = char;
+    using T2 = std::char_traits<C2>;
+    using A2 = std::allocator<C2>;
+
+    return string_conv_impl<C, T, A, C2, T2, A2>::template invoke<N>(s);
+}
+
+} // namespace detail
+} // namespace toml
+#endif // TOML11_UTILITY_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/value.hpp b/src/frontend/qt_sdl/toml/toml11/value.hpp
new file mode 100644
index 00000000..d945fa01
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/value.hpp
@@ -0,0 +1,2257 @@
+#ifndef TOML11_VALUE_HPP
+#define TOML11_VALUE_HPP
+
+#include "color.hpp"
+#include "datetime.hpp"
+#include "exception.hpp"
+#include "error_info.hpp"
+#include "format.hpp"
+#include "region.hpp"
+#include "source_location.hpp"
+#include "storage.hpp"
+#include "traits.hpp"
+#include "value_t.hpp"
+#include "version.hpp" // IWYU pragma: keep < TOML11_HAS_STRING_VIEW
+
+#ifdef TOML11_HAS_STRING_VIEW
+#include <string_view>
+#endif
+
+#include <cassert>
+
+namespace toml
+{
+template<typename TypeConfig>
+class basic_value;
+
+struct type_error final : public ::toml::exception
+{
+  public:
+    type_error(std::string what_arg, source_location loc)
+        : what_(std::move(what_arg)), loc_(std::move(loc))
+    {}
+    ~type_error() noexcept override = default;
+
+    const char* what() const noexcept override {return what_.c_str();}
+
+    source_location const& location() const noexcept {return loc_;}
+
+  private:
+    std::string what_;
+    source_location loc_;
+};
+
+// only for internal use
+namespace detail
+{
+template<typename TC>
+error_info make_type_error(const basic_value<TC>&, const std::string&, const value_t);
+
+template<typename TC>
+error_info make_not_found_error(const basic_value<TC>&, const std::string&, const typename basic_value<TC>::key_type&);
+
+template<typename TC>
+void change_region_of_value(basic_value<TC>&, const basic_value<TC>&);
+
+template<typename TC, value_t V>
+struct getter;
+} // detail
+
+template<typename TypeConfig>
+class basic_value
+{
+  public:
+
+    using config_type          = TypeConfig;
+    using key_type             = typename config_type::string_type;
+    using value_type           = basic_value<config_type>;
+    using boolean_type         = typename config_type::boolean_type;
+    using integer_type         = typename config_type::integer_type;
+    using floating_type        = typename config_type::floating_type;
+    using string_type          = typename config_type::string_type;
+    using local_time_type      = ::toml::local_time;
+    using local_date_type      = ::toml::local_date;
+    using local_datetime_type  = ::toml::local_datetime;
+    using offset_datetime_type = ::toml::offset_datetime;
+    using array_type           = typename config_type::template array_type<value_type>;
+    using table_type           = typename config_type::template table_type<key_type, value_type>;
+    using comment_type         = typename config_type::comment_type;
+    using char_type            = typename string_type::value_type;
+
+  private:
+
+    using region_type = detail::region;
+
+  public:
+
+    basic_value() noexcept
+        : type_(value_t::empty), empty_('\0'), region_{}, comments_{}
+    {}
+    ~basic_value() noexcept {this->cleanup();}
+
+    // copy/move constructor/assigner ===================================== {{{
+
+    basic_value(const basic_value& v)
+        : type_(v.type_), region_(v.region_), comments_(v.comments_)
+    {
+        switch(this->type_)
+        {
+            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
+            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
+            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
+            case value_t::string         : assigner(string_         , v.string_         ); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
+            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
+            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
+            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
+            case value_t::array          : assigner(array_          , v.array_          ); break;
+            case value_t::table          : assigner(table_          , v.table_          ); break;
+            default                      : assigner(empty_          , '\0'              ); break;
+        }
+    }
+    basic_value(basic_value&& v)
+        : type_(v.type()), region_(std::move(v.region_)),
+          comments_(std::move(v.comments_))
+    {
+        switch(this->type_)
+        {
+            case value_t::boolean        : assigner(boolean_        , std::move(v.boolean_        )); break;
+            case value_t::integer        : assigner(integer_        , std::move(v.integer_        )); break;
+            case value_t::floating       : assigner(floating_       , std::move(v.floating_       )); break;
+            case value_t::string         : assigner(string_         , std::move(v.string_         )); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, std::move(v.offset_datetime_)); break;
+            case value_t::local_datetime : assigner(local_datetime_ , std::move(v.local_datetime_ )); break;
+            case value_t::local_date     : assigner(local_date_     , std::move(v.local_date_     )); break;
+            case value_t::local_time     : assigner(local_time_     , std::move(v.local_time_     )); break;
+            case value_t::array          : assigner(array_          , std::move(v.array_          )); break;
+            case value_t::table          : assigner(table_          , std::move(v.table_          )); break;
+            default                      : assigner(empty_          , '\0'                         ); break;
+        }
+    }
+
+    basic_value& operator=(const basic_value& v)
+    {
+        if(this == std::addressof(v)) {return *this;}
+
+        this->cleanup();
+        this->type_     = v.type_;
+        this->region_   = v.region_;
+        this->comments_ = v.comments_;
+        switch(this->type_)
+        {
+            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
+            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
+            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
+            case value_t::string         : assigner(string_         , v.string_         ); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
+            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
+            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
+            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
+            case value_t::array          : assigner(array_          , v.array_          ); break;
+            case value_t::table          : assigner(table_          , v.table_          ); break;
+            default                      : assigner(empty_          , '\0'              ); break;
+        }
+        return *this;
+    }
+    basic_value& operator=(basic_value&& v)
+    {
+        if(this == std::addressof(v)) {return *this;}
+
+        this->cleanup();
+        this->type_     = v.type_;
+        this->region_   = std::move(v.region_);
+        this->comments_ = std::move(v.comments_);
+        switch(this->type_)
+        {
+            case value_t::boolean        : assigner(boolean_        , std::move(v.boolean_        )); break;
+            case value_t::integer        : assigner(integer_        , std::move(v.integer_        )); break;
+            case value_t::floating       : assigner(floating_       , std::move(v.floating_       )); break;
+            case value_t::string         : assigner(string_         , std::move(v.string_         )); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, std::move(v.offset_datetime_)); break;
+            case value_t::local_datetime : assigner(local_datetime_ , std::move(v.local_datetime_ )); break;
+            case value_t::local_date     : assigner(local_date_     , std::move(v.local_date_     )); break;
+            case value_t::local_time     : assigner(local_time_     , std::move(v.local_time_     )); break;
+            case value_t::array          : assigner(array_          , std::move(v.array_          )); break;
+            case value_t::table          : assigner(table_          , std::move(v.table_          )); break;
+            default                      : assigner(empty_          , '\0'                         ); break;
+        }
+        return *this;
+    }
+    // }}}
+
+    // constructor to overwrite commnets ================================== {{{
+
+    basic_value(basic_value v, std::vector<std::string> com)
+        : type_(v.type()), region_(std::move(v.region_)),
+          comments_(std::move(com))
+    {
+        switch(this->type_)
+        {
+            case value_t::boolean        : assigner(boolean_        , std::move(v.boolean_        )); break;
+            case value_t::integer        : assigner(integer_        , std::move(v.integer_        )); break;
+            case value_t::floating       : assigner(floating_       , std::move(v.floating_       )); break;
+            case value_t::string         : assigner(string_         , std::move(v.string_         )); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, std::move(v.offset_datetime_)); break;
+            case value_t::local_datetime : assigner(local_datetime_ , std::move(v.local_datetime_ )); break;
+            case value_t::local_date     : assigner(local_date_     , std::move(v.local_date_     )); break;
+            case value_t::local_time     : assigner(local_time_     , std::move(v.local_time_     )); break;
+            case value_t::array          : assigner(array_          , std::move(v.array_          )); break;
+            case value_t::table          : assigner(table_          , std::move(v.table_          )); break;
+            default                      : assigner(empty_          , '\0'                         ); break;
+        }
+    }
+    // }}}
+
+    // conversion between different basic_values ========================== {{{
+
+    template<typename TI>
+    basic_value(basic_value<TI> other)
+        : type_(other.type_),
+          region_(std::move(other.region_)),
+          comments_(std::move(other.comments_))
+    {
+        switch(other.type_)
+        {
+            // use auto-convert in constructor
+            case value_t::boolean        : assigner(boolean_        , std::move(other.boolean_        )); break;
+            case value_t::integer        : assigner(integer_        , std::move(other.integer_        )); break;
+            case value_t::floating       : assigner(floating_       , std::move(other.floating_       )); break;
+            case value_t::string         : assigner(string_         , std::move(other.string_         )); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, std::move(other.offset_datetime_)); break;
+            case value_t::local_datetime : assigner(local_datetime_ , std::move(other.local_datetime_ )); break;
+            case value_t::local_date     : assigner(local_date_     , std::move(other.local_date_     )); break;
+            case value_t::local_time     : assigner(local_time_     , std::move(other.local_time_     )); break;
+
+            // may have different container type
+            case value_t::array          :
+            {
+                array_type tmp(
+                    std::make_move_iterator(other.array_.value.get().begin()),
+                    std::make_move_iterator(other.array_.value.get().end()));
+                assigner(array_, array_storage(
+                        detail::storage<array_type>(std::move(tmp)),
+                        other.array_.format
+                    ));
+                break;
+            }
+            case value_t::table          :
+            {
+                table_type tmp(
+                    std::make_move_iterator(other.table_.value.get().begin()),
+                    std::make_move_iterator(other.table_.value.get().end()));
+                assigner(table_, table_storage(
+                        detail::storage<table_type>(std::move(tmp)),
+                        other.table_.format
+                    ));
+                break;
+            }
+            default: break;
+        }
+    }
+
+    template<typename TI>
+    basic_value(basic_value<TI> other, std::vector<std::string> com)
+        : type_(other.type_),
+          region_(std::move(other.region_)),
+          comments_(std::move(com))
+    {
+        switch(other.type_)
+        {
+            // use auto-convert in constructor
+            case value_t::boolean        : assigner(boolean_        , std::move(other.boolean_        )); break;
+            case value_t::integer        : assigner(integer_        , std::move(other.integer_        )); break;
+            case value_t::floating       : assigner(floating_       , std::move(other.floating_       )); break;
+            case value_t::string         : assigner(string_         , std::move(other.string_         )); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, std::move(other.offset_datetime_)); break;
+            case value_t::local_datetime : assigner(local_datetime_ , std::move(other.local_datetime_ )); break;
+            case value_t::local_date     : assigner(local_date_     , std::move(other.local_date_     )); break;
+            case value_t::local_time     : assigner(local_time_     , std::move(other.local_time_     )); break;
+
+            // may have different container type
+            case value_t::array          :
+            {
+                array_type tmp(
+                    std::make_move_iterator(other.array_.value.get().begin()),
+                    std::make_move_iterator(other.array_.value.get().end()));
+                assigner(array_, array_storage(
+                        detail::storage<array_type>(std::move(tmp)),
+                        other.array_.format
+                    ));
+                break;
+            }
+            case value_t::table          :
+            {
+                table_type tmp(
+                    std::make_move_iterator(other.table_.value.get().begin()),
+                    std::make_move_iterator(other.table_.value.get().end()));
+                assigner(table_, table_storage(
+                        detail::storage<table_type>(std::move(tmp)),
+                        other.table_.format
+                    ));
+                break;
+            }
+            default: break;
+        }
+    }
+    template<typename TI>
+    basic_value& operator=(basic_value<TI> other)
+    {
+        this->cleanup();
+        this->region_ = other.region_;
+        this->comments_    = comment_type(other.comments_);
+        this->type_        = other.type_;
+        switch(other.type_)
+        {
+            // use auto-convert in constructor
+            case value_t::boolean        : assigner(boolean_        , std::move(other.boolean_        )); break;
+            case value_t::integer        : assigner(integer_        , std::move(other.integer_        )); break;
+            case value_t::floating       : assigner(floating_       , std::move(other.floating_       )); break;
+            case value_t::string         : assigner(string_         , std::move(other.string_         )); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, std::move(other.offset_datetime_)); break;
+            case value_t::local_datetime : assigner(local_datetime_ , std::move(other.local_datetime_ )); break;
+            case value_t::local_date     : assigner(local_date_     , std::move(other.local_date_     )); break;
+            case value_t::local_time     : assigner(local_time_     , std::move(other.local_time_     )); break;
+
+            // may have different container type
+            case value_t::array          :
+            {
+                array_type tmp(
+                    std::make_move_iterator(other.array_.value.get().begin()),
+                    std::make_move_iterator(other.array_.value.get().end()));
+                assigner(array_, array_storage(
+                        detail::storage<array_type>(std::move(tmp)),
+                        other.array_.format
+                    ));
+                break;
+            }
+            case value_t::table          :
+            {
+                table_type tmp(
+                    std::make_move_iterator(other.table_.value.get().begin()),
+                    std::make_move_iterator(other.table_.value.get().end()));
+                assigner(table_, table_storage(
+                        detail::storage<table_type>(std::move(tmp)),
+                        other.table_.format
+                    ));
+                break;
+            }
+            default: break;
+        }
+        return *this;
+    }
+    // }}}
+
+    // constructor (boolean) ============================================== {{{
+
+    basic_value(boolean_type x)
+        : basic_value(x, boolean_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(boolean_type x, boolean_format_info fmt)
+        : basic_value(x, fmt, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(boolean_type x, std::vector<std::string> com)
+        : basic_value(x, boolean_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(boolean_type x, boolean_format_info fmt, std::vector<std::string> com)
+        : basic_value(x, fmt, std::move(com), region_type{})
+    {}
+    basic_value(boolean_type x, boolean_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::boolean), boolean_(boolean_storage(x, fmt)),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(boolean_type x)
+    {
+        boolean_format_info fmt;
+        if(this->is_boolean())
+        {
+            fmt = this->as_boolean_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::boolean;
+        this->region_ = region_type{};
+        assigner(this->boolean_, boolean_storage(x, fmt));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (integer) ============================================== {{{
+
+    basic_value(integer_type x)
+        : basic_value(std::move(x), integer_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(integer_type x, integer_format_info fmt)
+        : basic_value(std::move(x), std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(integer_type x, std::vector<std::string> com)
+        : basic_value(std::move(x), integer_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(integer_type x, integer_format_info fmt, std::vector<std::string> com)
+        : basic_value(std::move(x), std::move(fmt), std::move(com), region_type{})
+    {}
+    basic_value(integer_type x, integer_format_info fmt, std::vector<std::string> com, region_type reg)
+        : type_(value_t::integer), integer_(integer_storage(std::move(x), std::move(fmt))),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(integer_type x)
+    {
+        integer_format_info fmt;
+        if(this->is_integer())
+        {
+            fmt = this->as_integer_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::integer;
+        this->region_ = region_type{};
+        assigner(this->integer_, integer_storage(std::move(x), std::move(fmt)));
+        return *this;
+    }
+
+  private:
+
+    template<typename T>
+    using enable_if_integer_like_t = cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<T>, boolean_type>>,
+            cxx::negation<std::is_same<cxx::remove_cvref_t<T>, integer_type>>,
+            std::is_integral<cxx::remove_cvref_t<T>>
+        >::value, std::nullptr_t>;
+
+  public:
+
+    template<typename T, enable_if_integer_like_t<T> = nullptr>
+    basic_value(T x)
+        : basic_value(std::move(x), integer_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    template<typename T, enable_if_integer_like_t<T> = nullptr>
+    basic_value(T x, integer_format_info fmt)
+        : basic_value(std::move(x), std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    template<typename T, enable_if_integer_like_t<T> = nullptr>
+    basic_value(T x, std::vector<std::string> com)
+        : basic_value(std::move(x), integer_format_info{}, std::move(com), region_type{})
+    {}
+    template<typename T, enable_if_integer_like_t<T> = nullptr>
+    basic_value(T x, integer_format_info fmt, std::vector<std::string> com)
+        : basic_value(std::move(x), std::move(fmt), std::move(com), region_type{})
+    {}
+    template<typename T, enable_if_integer_like_t<T> = nullptr>
+    basic_value(T x, integer_format_info fmt, std::vector<std::string> com, region_type reg)
+        : type_(value_t::integer), integer_(integer_storage(std::move(x), std::move(fmt))),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    template<typename T, enable_if_integer_like_t<T> = nullptr>
+    basic_value& operator=(T x)
+    {
+        integer_format_info fmt;
+        if(this->is_integer())
+        {
+            fmt = this->as_integer_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::integer;
+        this->region_ = region_type{};
+        assigner(this->integer_, integer_storage(x, std::move(fmt)));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (floating) ============================================= {{{
+
+    basic_value(floating_type x)
+        : basic_value(std::move(x), floating_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(floating_type x, floating_format_info fmt)
+        : basic_value(std::move(x), std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(floating_type x, std::vector<std::string> com)
+        : basic_value(std::move(x), floating_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(floating_type x, floating_format_info fmt, std::vector<std::string> com)
+        : basic_value(std::move(x), std::move(fmt), std::move(com), region_type{})
+    {}
+    basic_value(floating_type x, floating_format_info fmt, std::vector<std::string> com, region_type reg)
+        : type_(value_t::floating), floating_(floating_storage(std::move(x), std::move(fmt))),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(floating_type x)
+    {
+        floating_format_info fmt;
+        if(this->is_floating())
+        {
+            fmt = this->as_floating_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::floating;
+        this->region_ = region_type{};
+        assigner(this->floating_, floating_storage(std::move(x), std::move(fmt)));
+        return *this;
+    }
+
+  private:
+
+    template<typename T>
+    using enable_if_floating_like_t = cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<T>, floating_type>>,
+            std::is_floating_point<cxx::remove_cvref_t<T>>
+        >::value, std::nullptr_t>;
+
+  public:
+
+    template<typename T, enable_if_floating_like_t<T> = nullptr>
+    basic_value(T x)
+        : basic_value(x, floating_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+
+    template<typename T, enable_if_floating_like_t<T> = nullptr>
+    basic_value(T x, floating_format_info fmt)
+        : basic_value(x, std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+
+    template<typename T, enable_if_floating_like_t<T> = nullptr>
+    basic_value(T x, std::vector<std::string> com)
+        : basic_value(x, floating_format_info{}, std::move(com), region_type{})
+    {}
+
+    template<typename T, enable_if_floating_like_t<T> = nullptr>
+    basic_value(T x, floating_format_info fmt, std::vector<std::string> com)
+        : basic_value(x, std::move(fmt), std::move(com), region_type{})
+    {}
+
+    template<typename T, enable_if_floating_like_t<T> = nullptr>
+    basic_value(T x, floating_format_info fmt, std::vector<std::string> com, region_type reg)
+        : type_(value_t::floating), floating_(floating_storage(x, std::move(fmt))),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+
+    template<typename T, enable_if_floating_like_t<T> = nullptr>
+    basic_value& operator=(T x)
+    {
+        floating_format_info fmt;
+        if(this->is_floating())
+        {
+            fmt = this->as_floating_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::floating;
+        this->region_ = region_type{};
+        assigner(this->floating_, floating_storage(x, std::move(fmt)));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (string) =============================================== {{{
+
+    basic_value(string_type x)
+        : basic_value(std::move(x), string_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(string_type x, string_format_info fmt)
+        : basic_value(std::move(x), std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(string_type x, std::vector<std::string> com)
+        : basic_value(std::move(x), string_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(string_type x, string_format_info fmt, std::vector<std::string> com)
+        : basic_value(std::move(x), std::move(fmt), std::move(com), region_type{})
+    {}
+    basic_value(string_type x, string_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::string), string_(string_storage(std::move(x), std::move(fmt))),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(string_type x)
+    {
+        string_format_info fmt;
+        if(this->is_string())
+        {
+            fmt = this->as_string_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::string;
+        this->region_ = region_type{};
+        assigner(this->string_, string_storage(x, std::move(fmt)));
+        return *this;
+    }
+
+    // "string literal"
+
+    basic_value(const typename string_type::value_type* x)
+        : basic_value(x, string_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(const typename string_type::value_type* x, string_format_info fmt)
+        : basic_value(x, std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(const typename string_type::value_type* x, std::vector<std::string> com)
+        : basic_value(x, string_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(const typename string_type::value_type* x, string_format_info fmt, std::vector<std::string> com)
+        : basic_value(x, std::move(fmt), std::move(com), region_type{})
+    {}
+    basic_value(const typename string_type::value_type* x, string_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::string), string_(string_storage(string_type(x), std::move(fmt))),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(const typename string_type::value_type* x)
+    {
+        string_format_info fmt;
+        if(this->is_string())
+        {
+            fmt = this->as_string_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::string;
+        this->region_ = region_type{};
+        assigner(this->string_, string_storage(string_type(x), std::move(fmt)));
+        return *this;
+    }
+
+#if defined(TOML11_HAS_STRING_VIEW)
+    using string_view_type = std::basic_string_view<
+        typename string_type::value_type, typename string_type::traits_type>;
+
+    basic_value(string_view_type x)
+        : basic_value(x, string_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(string_view_type x, string_format_info fmt)
+        : basic_value(x, std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(string_view_type x, std::vector<std::string> com)
+        : basic_value(x, string_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(string_view_type x, string_format_info fmt, std::vector<std::string> com)
+        : basic_value(x, std::move(fmt), std::move(com), region_type{})
+    {}
+    basic_value(string_view_type x, string_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::string), string_(string_storage(string_type(x), std::move(fmt))),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(string_view_type x)
+    {
+        string_format_info fmt;
+        if(this->is_string())
+        {
+            fmt = this->as_string_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::string;
+        this->region_ = region_type{};
+        assigner(this->string_, string_storage(string_type(x), std::move(fmt)));
+        return *this;
+    }
+
+#endif // TOML11_HAS_STRING_VIEW
+
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<T>, string_type>>,
+            detail::is_1byte_std_basic_string<T>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value(const T& x)
+        : basic_value(x, string_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<T>, string_type>>,
+            detail::is_1byte_std_basic_string<T>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value(const T& x, string_format_info fmt)
+        : basic_value(x, std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<T>, string_type>>,
+            detail::is_1byte_std_basic_string<T>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value(const T& x, std::vector<std::string> com)
+        : basic_value(x, string_format_info{}, std::move(com), region_type{})
+    {}
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<T>, string_type>>,
+            detail::is_1byte_std_basic_string<T>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value(const T& x, string_format_info fmt, std::vector<std::string> com)
+        : basic_value(x, std::move(fmt), std::move(com), region_type{})
+    {}
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<T>, string_type>>,
+            detail::is_1byte_std_basic_string<T>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value(const T& x, string_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::string),
+          string_(string_storage(detail::string_conv<string_type>(x), std::move(fmt))),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<cxx::remove_cvref_t<T>, string_type>>,
+            detail::is_1byte_std_basic_string<T>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value& operator=(const T& x)
+    {
+        string_format_info fmt;
+        if(this->is_string())
+        {
+            fmt = this->as_string_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::string;
+        this->region_ = region_type{};
+        assigner(this->string_, string_storage(detail::string_conv<string_type>(x), std::move(fmt)));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (local_date) =========================================== {{{
+
+    basic_value(local_date_type x)
+        : basic_value(x, local_date_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(local_date_type x, local_date_format_info fmt)
+        : basic_value(x, fmt, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(local_date_type x, std::vector<std::string> com)
+        : basic_value(x, local_date_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(local_date_type x, local_date_format_info fmt, std::vector<std::string> com)
+        : basic_value(x, fmt, std::move(com), region_type{})
+    {}
+    basic_value(local_date_type x, local_date_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::local_date), local_date_(local_date_storage(x, fmt)),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(local_date_type x)
+    {
+        local_date_format_info fmt;
+        if(this->is_local_date())
+        {
+            fmt = this->as_local_date_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::local_date;
+        this->region_ = region_type{};
+        assigner(this->local_date_, local_date_storage(x, fmt));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (local_time) =========================================== {{{
+
+    basic_value(local_time_type x)
+        : basic_value(x, local_time_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(local_time_type x, local_time_format_info fmt)
+        : basic_value(x, fmt, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(local_time_type x, std::vector<std::string> com)
+        : basic_value(x, local_time_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(local_time_type x, local_time_format_info fmt, std::vector<std::string> com)
+        : basic_value(x, fmt, std::move(com), region_type{})
+    {}
+    basic_value(local_time_type x, local_time_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::local_time), local_time_(local_time_storage(x, fmt)),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(local_time_type x)
+    {
+        local_time_format_info fmt;
+        if(this->is_local_time())
+        {
+            fmt = this->as_local_time_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::local_time;
+        this->region_ = region_type{};
+        assigner(this->local_time_, local_time_storage(x, fmt));
+        return *this;
+    }
+
+    template<typename Rep, typename Period>
+    basic_value(const std::chrono::duration<Rep, Period>& x)
+        : basic_value(local_time_type(x), local_time_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    template<typename Rep, typename Period>
+    basic_value(const std::chrono::duration<Rep, Period>& x, local_time_format_info fmt)
+        : basic_value(local_time_type(x), std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    template<typename Rep, typename Period>
+    basic_value(const std::chrono::duration<Rep, Period>& x, std::vector<std::string> com)
+        : basic_value(local_time_type(x), local_time_format_info{}, std::move(com), region_type{})
+    {}
+    template<typename Rep, typename Period>
+    basic_value(const std::chrono::duration<Rep, Period>& x, local_time_format_info fmt, std::vector<std::string> com)
+        : basic_value(local_time_type(x), std::move(fmt), std::move(com), region_type{})
+    {}
+    template<typename Rep, typename Period>
+    basic_value(const std::chrono::duration<Rep, Period>& x,
+                local_time_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : basic_value(local_time_type(x), std::move(fmt), std::move(com), std::move(reg))
+    {}
+    template<typename Rep, typename Period>
+    basic_value& operator=(const std::chrono::duration<Rep, Period>& x)
+    {
+        local_time_format_info fmt;
+        if(this->is_local_time())
+        {
+            fmt = this->as_local_time_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::local_time;
+        this->region_ = region_type{};
+        assigner(this->local_time_, local_time_storage(local_time_type(x), std::move(fmt)));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (local_datetime) =========================================== {{{
+
+    basic_value(local_datetime_type x)
+        : basic_value(x, local_datetime_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(local_datetime_type x, local_datetime_format_info fmt)
+        : basic_value(x, fmt, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(local_datetime_type x, std::vector<std::string> com)
+        : basic_value(x, local_datetime_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(local_datetime_type x, local_datetime_format_info fmt, std::vector<std::string> com)
+        : basic_value(x, fmt, std::move(com), region_type{})
+    {}
+    basic_value(local_datetime_type x, local_datetime_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::local_datetime), local_datetime_(local_datetime_storage(x, fmt)),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(local_datetime_type x)
+    {
+        local_datetime_format_info fmt;
+        if(this->is_local_datetime())
+        {
+            fmt = this->as_local_datetime_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::local_datetime;
+        this->region_ = region_type{};
+        assigner(this->local_datetime_, local_datetime_storage(x, fmt));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (offset_datetime) =========================================== {{{
+
+    basic_value(offset_datetime_type x)
+        : basic_value(x, offset_datetime_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(offset_datetime_type x, offset_datetime_format_info fmt)
+        : basic_value(x, fmt, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(offset_datetime_type x, std::vector<std::string> com)
+        : basic_value(x, offset_datetime_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(offset_datetime_type x, offset_datetime_format_info fmt, std::vector<std::string> com)
+        : basic_value(x, fmt, std::move(com), region_type{})
+    {}
+    basic_value(offset_datetime_type x, offset_datetime_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::offset_datetime), offset_datetime_(offset_datetime_storage(x, fmt)),
+          region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(offset_datetime_type x)
+    {
+        offset_datetime_format_info fmt;
+        if(this->is_offset_datetime())
+        {
+            fmt = this->as_offset_datetime_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::offset_datetime;
+        this->region_ = region_type{};
+        assigner(this->offset_datetime_, offset_datetime_storage(x, fmt));
+        return *this;
+    }
+
+    // system_clock::time_point
+
+    basic_value(std::chrono::system_clock::time_point x)
+        : basic_value(offset_datetime_type(x), offset_datetime_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(std::chrono::system_clock::time_point x, offset_datetime_format_info fmt)
+        : basic_value(offset_datetime_type(x), fmt, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(std::chrono::system_clock::time_point x, std::vector<std::string> com)
+        : basic_value(offset_datetime_type(x), offset_datetime_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(std::chrono::system_clock::time_point x, offset_datetime_format_info fmt, std::vector<std::string> com)
+        : basic_value(offset_datetime_type(x), fmt, std::move(com), region_type{})
+    {}
+    basic_value(std::chrono::system_clock::time_point x, offset_datetime_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : basic_value(offset_datetime_type(x), std::move(fmt), std::move(com), std::move(reg))
+    {}
+    basic_value& operator=(std::chrono::system_clock::time_point x)
+    {
+        offset_datetime_format_info fmt;
+        if(this->is_offset_datetime())
+        {
+            fmt = this->as_offset_datetime_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::offset_datetime;
+        this->region_ = region_type{};
+        assigner(this->offset_datetime_, offset_datetime_storage(offset_datetime_type(x), fmt));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (array) ================================================ {{{
+
+    basic_value(array_type x)
+        : basic_value(std::move(x), array_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(array_type x, array_format_info fmt)
+        : basic_value(std::move(x), std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(array_type x, std::vector<std::string> com)
+        : basic_value(std::move(x), array_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(array_type x, array_format_info fmt, std::vector<std::string> com)
+        : basic_value(std::move(x), fmt, std::move(com), region_type{})
+    {}
+    basic_value(array_type x, array_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::array), array_(array_storage(
+              detail::storage<array_type>(std::move(x)), std::move(fmt)
+          )), region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(array_type x)
+    {
+        array_format_info fmt;
+        if(this->is_array())
+        {
+            fmt = this->as_array_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::array;
+        this->region_ = region_type{};
+        assigner(this->array_, array_storage(
+                    detail::storage<array_type>(std::move(x)), std::move(fmt)));
+        return *this;
+    }
+
+  private:
+
+    template<typename T>
+    using enable_if_array_like_t = cxx::enable_if_t<cxx::conjunction<
+            detail::is_container<T>,
+            cxx::negation<std::is_same<T, array_type>>,
+            cxx::negation<detail::is_std_basic_string<T>>,
+#if defined(TOML11_HAS_STRING_VIEW)
+            cxx::negation<detail::is_std_basic_string_view<T>>,
+#endif
+            cxx::negation<detail::has_from_toml_method<T, config_type>>,
+            cxx::negation<detail::has_specialized_from<T>>
+        >::value, std::nullptr_t>;
+
+  public:
+
+    template<typename T, enable_if_array_like_t<T> = nullptr>
+    basic_value(T x)
+        : basic_value(std::move(x), array_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    template<typename T, enable_if_array_like_t<T> = nullptr>
+    basic_value(T x, array_format_info fmt)
+        : basic_value(std::move(x), std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    template<typename T, enable_if_array_like_t<T> = nullptr>
+    basic_value(T x, std::vector<std::string> com)
+        : basic_value(std::move(x), array_format_info{}, std::move(com), region_type{})
+    {}
+    template<typename T, enable_if_array_like_t<T> = nullptr>
+    basic_value(T x, array_format_info fmt, std::vector<std::string> com)
+        : basic_value(std::move(x), fmt, std::move(com), region_type{})
+    {}
+    template<typename T, enable_if_array_like_t<T> = nullptr>
+    basic_value(T x, array_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::array), array_(array_storage(
+              detail::storage<array_type>(array_type(
+                      std::make_move_iterator(x.begin()),
+                      std::make_move_iterator(x.end()))
+              ), std::move(fmt)
+          )), region_(std::move(reg)), comments_(std::move(com))
+    {}
+    template<typename T, enable_if_array_like_t<T> = nullptr>
+    basic_value& operator=(T x)
+    {
+        array_format_info fmt;
+        if(this->is_array())
+        {
+            fmt = this->as_array_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::array;
+        this->region_ = region_type{};
+
+        array_type a(std::make_move_iterator(x.begin()),
+                     std::make_move_iterator(x.end()));
+        assigner(this->array_, array_storage(
+                    detail::storage<array_type>(std::move(a)), std::move(fmt)));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (table) ================================================ {{{
+
+    basic_value(table_type x)
+        : basic_value(std::move(x), table_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(table_type x, table_format_info fmt)
+        : basic_value(std::move(x), std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    basic_value(table_type x, std::vector<std::string> com)
+        : basic_value(std::move(x), table_format_info{}, std::move(com), region_type{})
+    {}
+    basic_value(table_type x, table_format_info fmt, std::vector<std::string> com)
+        : basic_value(std::move(x), fmt, std::move(com), region_type{})
+    {}
+    basic_value(table_type x, table_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::table), table_(table_storage(
+                detail::storage<table_type>(std::move(x)), std::move(fmt)
+          )), region_(std::move(reg)), comments_(std::move(com))
+    {}
+    basic_value& operator=(table_type x)
+    {
+        table_format_info fmt;
+        if(this->is_table())
+        {
+            fmt = this->as_table_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::table;
+        this->region_ = region_type{};
+        assigner(this->table_, table_storage(
+            detail::storage<table_type>(std::move(x)), std::move(fmt)));
+        return *this;
+    }
+
+    // table-like
+
+  private:
+
+    template<typename T>
+    using enable_if_table_like_t = cxx::enable_if_t<cxx::conjunction<
+            cxx::negation<std::is_same<T, table_type>>,
+            detail::is_map<T>,
+            cxx::negation<detail::has_from_toml_method<T, config_type>>,
+            cxx::negation<detail::has_specialized_from<T>>
+        >::value, std::nullptr_t>;
+
+  public:
+
+    template<typename T, enable_if_table_like_t<T> = nullptr>
+    basic_value(T x)
+        : basic_value(std::move(x), table_format_info{}, std::vector<std::string>{}, region_type{})
+    {}
+    template<typename T, enable_if_table_like_t<T> = nullptr>
+    basic_value(T x, table_format_info fmt)
+        : basic_value(std::move(x), std::move(fmt), std::vector<std::string>{}, region_type{})
+    {}
+    template<typename T, enable_if_table_like_t<T> = nullptr>
+    basic_value(T x, std::vector<std::string> com)
+        : basic_value(std::move(x), table_format_info{}, std::move(com), region_type{})
+    {}
+    template<typename T, enable_if_table_like_t<T> = nullptr>
+    basic_value(T x, table_format_info fmt, std::vector<std::string> com)
+        : basic_value(std::move(x), fmt, std::move(com), region_type{})
+    {}
+    template<typename T, enable_if_table_like_t<T> = nullptr>
+    basic_value(T x, table_format_info fmt,
+                std::vector<std::string> com, region_type reg)
+        : type_(value_t::table), table_(table_storage(
+              detail::storage<table_type>(table_type(
+                      std::make_move_iterator(x.begin()),
+                      std::make_move_iterator(x.end())
+              )), std::move(fmt)
+          )), region_(std::move(reg)), comments_(std::move(com))
+    {}
+    template<typename T, enable_if_table_like_t<T> = nullptr>
+    basic_value& operator=(T x)
+    {
+        table_format_info fmt;
+        if(this->is_table())
+        {
+            fmt = this->as_table_fmt();
+        }
+        this->cleanup();
+        this->type_   = value_t::table;
+        this->region_ = region_type{};
+
+        table_type t(std::make_move_iterator(x.begin()),
+                     std::make_move_iterator(x.end()));
+        assigner(this->table_, table_storage(
+            detail::storage<table_type>(std::move(t)), std::move(fmt)));
+        return *this;
+    }
+
+    // }}}
+
+    // constructor (user_defined) ========================================= {{{
+
+    template<typename T, cxx::enable_if_t<
+        detail::has_specialized_into<T>::value, std::nullptr_t> = nullptr>
+    basic_value(const T& ud)
+        : basic_value(
+            into<cxx::remove_cvref_t<T>>::template into_toml<config_type>(ud))
+    {}
+    template<typename T, cxx::enable_if_t<
+        detail::has_specialized_into<T>::value, std::nullptr_t> = nullptr>
+    basic_value(const T& ud, std::vector<std::string> com)
+        : basic_value(
+            into<cxx::remove_cvref_t<T>>::template into_toml<config_type>(ud),
+            std::move(com))
+    {}
+    template<typename T, cxx::enable_if_t<
+        detail::has_specialized_into<T>::value, std::nullptr_t> = nullptr>
+    basic_value& operator=(const T& ud)
+    {
+        *this = into<cxx::remove_cvref_t<T>>::template into_toml<config_type>(ud);
+        return *this;
+    }
+
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            detail::has_into_toml_method<T>,
+            cxx::negation<detail::has_specialized_into<T>>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value(const T& ud): basic_value(ud.into_toml()) {}
+
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            detail::has_into_toml_method<T>,
+            cxx::negation<detail::has_specialized_into<T>>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value(const T& ud, std::vector<std::string> com)
+        : basic_value(ud.into_toml(), std::move(com))
+    {}
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            detail::has_into_toml_method<T>,
+            cxx::negation<detail::has_specialized_into<T>>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value& operator=(const T& ud)
+    {
+        *this = ud.into_toml();
+        return *this;
+    }
+
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            detail::has_template_into_toml_method<T, TypeConfig>,
+            cxx::negation<detail::has_specialized_into<T>>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value(const T& ud): basic_value(ud.template into_toml<TypeConfig>()) {}
+
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            detail::has_template_into_toml_method<T, TypeConfig>,
+            cxx::negation<detail::has_specialized_into<T>>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value(const T& ud, std::vector<std::string> com)
+        : basic_value(ud.template into_toml<TypeConfig>(), std::move(com))
+    {}
+    template<typename T, cxx::enable_if_t<cxx::conjunction<
+            detail::has_template_into_toml_method<T, TypeConfig>,
+            cxx::negation<detail::has_specialized_into<T>>
+        >::value, std::nullptr_t> = nullptr>
+    basic_value& operator=(const T& ud)
+    {
+        *this = ud.template into_toml<TypeConfig>();
+        return *this;
+    }
+    // }}}
+
+    // empty value with region info ======================================= {{{
+
+    // mainly for `null` extension
+    basic_value(detail::none_t, region_type reg) noexcept
+        : type_(value_t::empty), empty_('\0'), region_(std::move(reg)), comments_{}
+    {}
+
+    // }}}
+
+    // type checking ====================================================== {{{
+
+    template<typename T, cxx::enable_if_t<
+        detail::is_exact_toml_type<cxx::remove_cvref_t<T>, value_type>::value,
+        std::nullptr_t> = nullptr>
+    bool is() const noexcept
+    {
+        return detail::type_to_enum<T, value_type>::value == this->type_;
+    }
+    bool is(value_t t) const noexcept {return t == this->type_;}
+
+    bool is_empty()           const noexcept {return this->is(value_t::empty          );}
+    bool is_boolean()         const noexcept {return this->is(value_t::boolean        );}
+    bool is_integer()         const noexcept {return this->is(value_t::integer        );}
+    bool is_floating()        const noexcept {return this->is(value_t::floating       );}
+    bool is_string()          const noexcept {return this->is(value_t::string         );}
+    bool is_offset_datetime() const noexcept {return this->is(value_t::offset_datetime);}
+    bool is_local_datetime()  const noexcept {return this->is(value_t::local_datetime );}
+    bool is_local_date()      const noexcept {return this->is(value_t::local_date     );}
+    bool is_local_time()      const noexcept {return this->is(value_t::local_time     );}
+    bool is_array()           const noexcept {return this->is(value_t::array          );}
+    bool is_table()           const noexcept {return this->is(value_t::table          );}
+
+    bool is_array_of_tables() const noexcept
+    {
+        if( ! this->is_array()) {return false;}
+        const auto& a = this->as_array(std::nothrow); // already checked.
+
+        // when you define [[array.of.tables]], at least one empty table will be
+        // assigned. In case of array of inline tables, `array_of_tables = []`,
+        // there is no reason to consider this as an array of *tables*.
+        // So empty array is not an array-of-tables.
+        if(a.empty()) {return false;}
+
+        // since toml v1.0.0 allows array of heterogeneous types, we need to
+        // check all the elements. if any of the elements is not a table, it
+        // is a heterogeneous array and cannot be expressed by `[[aot]]` form.
+        for(const auto& e : a)
+        {
+            if( ! e.is_table()) {return false;}
+        }
+        return true;
+    }
+
+    value_t type() const noexcept {return type_;}
+
+    // }}}
+
+    // as_xxx (noexcept) version ========================================== {{{
+
+    template<value_t T>
+    detail::enum_to_type_t<T, basic_value<config_type>> const&
+    as(const std::nothrow_t&) const noexcept
+    {
+        return detail::getter<config_type, T>::get_nothrow(*this);
+    }
+    template<value_t T>
+    detail::enum_to_type_t<T, basic_value<config_type>>&
+    as(const std::nothrow_t&) noexcept
+    {
+        return detail::getter<config_type, T>::get_nothrow(*this);
+    }
+
+    boolean_type         const& as_boolean        (const std::nothrow_t&) const noexcept {return this->boolean_.value;}
+    integer_type         const& as_integer        (const std::nothrow_t&) const noexcept {return this->integer_.value;}
+    floating_type        const& as_floating       (const std::nothrow_t&) const noexcept {return this->floating_.value;}
+    string_type          const& as_string         (const std::nothrow_t&) const noexcept {return this->string_.value;}
+    offset_datetime_type const& as_offset_datetime(const std::nothrow_t&) const noexcept {return this->offset_datetime_.value;}
+    local_datetime_type  const& as_local_datetime (const std::nothrow_t&) const noexcept {return this->local_datetime_.value;}
+    local_date_type      const& as_local_date     (const std::nothrow_t&) const noexcept {return this->local_date_.value;}
+    local_time_type      const& as_local_time     (const std::nothrow_t&) const noexcept {return this->local_time_.value;}
+    array_type           const& as_array          (const std::nothrow_t&) const noexcept {return this->array_.value.get();}
+    table_type           const& as_table          (const std::nothrow_t&) const noexcept {return this->table_.value.get();}
+
+    boolean_type        & as_boolean        (const std::nothrow_t&) noexcept {return this->boolean_.value;}
+    integer_type        & as_integer        (const std::nothrow_t&) noexcept {return this->integer_.value;}
+    floating_type       & as_floating       (const std::nothrow_t&) noexcept {return this->floating_.value;}
+    string_type         & as_string         (const std::nothrow_t&) noexcept {return this->string_.value;}
+    offset_datetime_type& as_offset_datetime(const std::nothrow_t&) noexcept {return this->offset_datetime_.value;}
+    local_datetime_type & as_local_datetime (const std::nothrow_t&) noexcept {return this->local_datetime_.value;}
+    local_date_type     & as_local_date     (const std::nothrow_t&) noexcept {return this->local_date_.value;}
+    local_time_type     & as_local_time     (const std::nothrow_t&) noexcept {return this->local_time_.value;}
+    array_type          & as_array          (const std::nothrow_t&) noexcept {return this->array_.value.get();}
+    table_type          & as_table          (const std::nothrow_t&) noexcept {return this->table_.value.get();}
+
+    // }}}
+
+    // as_xxx (throw) ===================================================== {{{
+
+    template<value_t T>
+    detail::enum_to_type_t<T, basic_value<config_type>> const& as() const
+    {
+        return detail::getter<config_type, T>::get(*this);
+    }
+    template<value_t T>
+    detail::enum_to_type_t<T, basic_value<config_type>>& as()
+    {
+        return detail::getter<config_type, T>::get(*this);
+    }
+
+    boolean_type const& as_boolean() const
+    {
+        if(this->type_ != value_t::boolean)
+        {
+            this->throw_bad_cast("toml::value::as_boolean()", value_t::boolean);
+        }
+        return this->boolean_.value;
+    }
+    integer_type const& as_integer() const
+    {
+        if(this->type_ != value_t::integer)
+        {
+            this->throw_bad_cast("toml::value::as_integer()", value_t::integer);
+        }
+        return this->integer_.value;
+    }
+    floating_type const& as_floating() const
+    {
+        if(this->type_ != value_t::floating)
+        {
+            this->throw_bad_cast("toml::value::as_floating()", value_t::floating);
+        }
+        return this->floating_.value;
+    }
+    string_type const& as_string() const
+    {
+        if(this->type_ != value_t::string)
+        {
+            this->throw_bad_cast("toml::value::as_string()", value_t::string);
+        }
+        return this->string_.value;
+    }
+    offset_datetime_type const& as_offset_datetime() const
+    {
+        if(this->type_ != value_t::offset_datetime)
+        {
+            this->throw_bad_cast("toml::value::as_offset_datetime()", value_t::offset_datetime);
+        }
+        return this->offset_datetime_.value;
+    }
+    local_datetime_type const& as_local_datetime() const
+    {
+        if(this->type_ != value_t::local_datetime)
+        {
+            this->throw_bad_cast("toml::value::as_local_datetime()", value_t::local_datetime);
+        }
+        return this->local_datetime_.value;
+    }
+    local_date_type const& as_local_date() const
+    {
+        if(this->type_ != value_t::local_date)
+        {
+            this->throw_bad_cast("toml::value::as_local_date()", value_t::local_date);
+        }
+        return this->local_date_.value;
+    }
+    local_time_type const& as_local_time() const
+    {
+        if(this->type_ != value_t::local_time)
+        {
+            this->throw_bad_cast("toml::value::as_local_time()", value_t::local_time);
+        }
+        return this->local_time_.value;
+    }
+    array_type const& as_array() const
+    {
+        if(this->type_ != value_t::array)
+        {
+            this->throw_bad_cast("toml::value::as_array()", value_t::array);
+        }
+        return this->array_.value.get();
+    }
+    table_type const& as_table() const
+    {
+        if(this->type_ != value_t::table)
+        {
+            this->throw_bad_cast("toml::value::as_table()", value_t::table);
+        }
+        return this->table_.value.get();
+    }
+
+    // ------------------------------------------------------------------------
+    // nonconst reference
+
+    boolean_type& as_boolean()
+    {
+        if(this->type_ != value_t::boolean)
+        {
+            this->throw_bad_cast("toml::value::as_boolean()", value_t::boolean);
+        }
+        return this->boolean_.value;
+    }
+    integer_type& as_integer()
+    {
+        if(this->type_ != value_t::integer)
+        {
+            this->throw_bad_cast("toml::value::as_integer()", value_t::integer);
+        }
+        return this->integer_.value;
+    }
+    floating_type& as_floating()
+    {
+        if(this->type_ != value_t::floating)
+        {
+            this->throw_bad_cast("toml::value::as_floating()", value_t::floating);
+        }
+        return this->floating_.value;
+    }
+    string_type& as_string()
+    {
+        if(this->type_ != value_t::string)
+        {
+            this->throw_bad_cast("toml::value::as_string()", value_t::string);
+        }
+        return this->string_.value;
+    }
+    offset_datetime_type& as_offset_datetime()
+    {
+        if(this->type_ != value_t::offset_datetime)
+        {
+            this->throw_bad_cast("toml::value::as_offset_datetime()", value_t::offset_datetime);
+        }
+        return this->offset_datetime_.value;
+    }
+    local_datetime_type& as_local_datetime()
+    {
+        if(this->type_ != value_t::local_datetime)
+        {
+            this->throw_bad_cast("toml::value::as_local_datetime()", value_t::local_datetime);
+        }
+        return this->local_datetime_.value;
+    }
+    local_date_type& as_local_date()
+    {
+        if(this->type_ != value_t::local_date)
+        {
+            this->throw_bad_cast("toml::value::as_local_date()", value_t::local_date);
+        }
+        return this->local_date_.value;
+    }
+    local_time_type& as_local_time()
+    {
+        if(this->type_ != value_t::local_time)
+        {
+            this->throw_bad_cast("toml::value::as_local_time()", value_t::local_time);
+        }
+        return this->local_time_.value;
+    }
+    array_type& as_array()
+    {
+        if(this->type_ != value_t::array)
+        {
+            this->throw_bad_cast("toml::value::as_array()", value_t::array);
+        }
+        return this->array_.value.get();
+    }
+    table_type& as_table()
+    {
+        if(this->type_ != value_t::table)
+        {
+            this->throw_bad_cast("toml::value::as_table()", value_t::table);
+        }
+        return this->table_.value.get();
+    }
+
+    // }}}
+
+    // format accessors (noexcept) ======================================== {{{
+
+    template<value_t T>
+    detail::enum_to_fmt_type_t<T> const&
+    as_fmt(const std::nothrow_t&) const noexcept
+    {
+        return detail::getter<config_type, T>::get_fmt_nothrow(*this);
+    }
+    template<value_t T>
+    detail::enum_to_fmt_type_t<T>&
+    as_fmt(const std::nothrow_t&) noexcept
+    {
+        return detail::getter<config_type, T>::get_fmt_nothrow(*this);
+    }
+
+    boolean_format_info        & as_boolean_fmt        (const std::nothrow_t&) noexcept {return this->boolean_.format;}
+    integer_format_info        & as_integer_fmt        (const std::nothrow_t&) noexcept {return this->integer_.format;}
+    floating_format_info       & as_floating_fmt       (const std::nothrow_t&) noexcept {return this->floating_.format;}
+    string_format_info         & as_string_fmt         (const std::nothrow_t&) noexcept {return this->string_.format;}
+    offset_datetime_format_info& as_offset_datetime_fmt(const std::nothrow_t&) noexcept {return this->offset_datetime_.format;}
+    local_datetime_format_info & as_local_datetime_fmt (const std::nothrow_t&) noexcept {return this->local_datetime_.format;}
+    local_date_format_info     & as_local_date_fmt     (const std::nothrow_t&) noexcept {return this->local_date_.format;}
+    local_time_format_info     & as_local_time_fmt     (const std::nothrow_t&) noexcept {return this->local_time_.format;}
+    array_format_info          & as_array_fmt          (const std::nothrow_t&) noexcept {return this->array_.format;}
+    table_format_info          & as_table_fmt          (const std::nothrow_t&) noexcept {return this->table_.format;}
+
+    boolean_format_info         const& as_boolean_fmt        (const std::nothrow_t&) const noexcept {return this->boolean_.format;}
+    integer_format_info         const& as_integer_fmt        (const std::nothrow_t&) const noexcept {return this->integer_.format;}
+    floating_format_info        const& as_floating_fmt       (const std::nothrow_t&) const noexcept {return this->floating_.format;}
+    string_format_info          const& as_string_fmt         (const std::nothrow_t&) const noexcept {return this->string_.format;}
+    offset_datetime_format_info const& as_offset_datetime_fmt(const std::nothrow_t&) const noexcept {return this->offset_datetime_.format;}
+    local_datetime_format_info  const& as_local_datetime_fmt (const std::nothrow_t&) const noexcept {return this->local_datetime_.format;}
+    local_date_format_info      const& as_local_date_fmt     (const std::nothrow_t&) const noexcept {return this->local_date_.format;}
+    local_time_format_info      const& as_local_time_fmt     (const std::nothrow_t&) const noexcept {return this->local_time_.format;}
+    array_format_info           const& as_array_fmt          (const std::nothrow_t&) const noexcept {return this->array_.format;}
+    table_format_info           const& as_table_fmt          (const std::nothrow_t&) const noexcept {return this->table_.format;}
+
+    // }}}
+
+    // format accessors (throw) =========================================== {{{
+
+    template<value_t T>
+    detail::enum_to_fmt_type_t<T> const& as_fmt() const
+    {
+        return detail::getter<config_type, T>::get_fmt(*this);
+    }
+    template<value_t T>
+    detail::enum_to_fmt_type_t<T>& as_fmt()
+    {
+        return detail::getter<config_type, T>::get_fmt(*this);
+    }
+
+    boolean_format_info const& as_boolean_fmt() const
+    {
+        if(this->type_ != value_t::boolean)
+        {
+            this->throw_bad_cast("toml::value::as_boolean_fmt()", value_t::boolean);
+        }
+        return this->boolean_.format;
+    }
+    integer_format_info const& as_integer_fmt() const
+    {
+        if(this->type_ != value_t::integer)
+        {
+            this->throw_bad_cast("toml::value::as_integer_fmt()", value_t::integer);
+        }
+        return this->integer_.format;
+    }
+    floating_format_info const& as_floating_fmt() const
+    {
+        if(this->type_ != value_t::floating)
+        {
+            this->throw_bad_cast("toml::value::as_floating_fmt()", value_t::floating);
+        }
+        return this->floating_.format;
+    }
+    string_format_info const& as_string_fmt() const
+    {
+        if(this->type_ != value_t::string)
+        {
+            this->throw_bad_cast("toml::value::as_string_fmt()", value_t::string);
+        }
+        return this->string_.format;
+    }
+    offset_datetime_format_info const& as_offset_datetime_fmt() const
+    {
+        if(this->type_ != value_t::offset_datetime)
+        {
+            this->throw_bad_cast("toml::value::as_offset_datetime_fmt()", value_t::offset_datetime);
+        }
+        return this->offset_datetime_.format;
+    }
+    local_datetime_format_info const& as_local_datetime_fmt() const
+    {
+        if(this->type_ != value_t::local_datetime)
+        {
+            this->throw_bad_cast("toml::value::as_local_datetime_fmt()", value_t::local_datetime);
+        }
+        return this->local_datetime_.format;
+    }
+    local_date_format_info const& as_local_date_fmt() const
+    {
+        if(this->type_ != value_t::local_date)
+        {
+            this->throw_bad_cast("toml::value::as_local_date_fmt()", value_t::local_date);
+        }
+        return this->local_date_.format;
+    }
+    local_time_format_info const& as_local_time_fmt() const
+    {
+        if(this->type_ != value_t::local_time)
+        {
+            this->throw_bad_cast("toml::value::as_local_time_fmt()", value_t::local_time);
+        }
+        return this->local_time_.format;
+    }
+    array_format_info const& as_array_fmt() const
+    {
+        if(this->type_ != value_t::array)
+        {
+            this->throw_bad_cast("toml::value::as_array_fmt()", value_t::array);
+        }
+        return this->array_.format;
+    }
+    table_format_info const& as_table_fmt() const
+    {
+        if(this->type_ != value_t::table)
+        {
+            this->throw_bad_cast("toml::value::as_table_fmt()", value_t::table);
+        }
+        return this->table_.format;
+    }
+
+    // ------------------------------------------------------------------------
+    // nonconst reference
+
+    boolean_format_info& as_boolean_fmt()
+    {
+        if(this->type_ != value_t::boolean)
+        {
+            this->throw_bad_cast("toml::value::as_boolean_fmt()", value_t::boolean);
+        }
+        return this->boolean_.format;
+    }
+    integer_format_info& as_integer_fmt()
+    {
+        if(this->type_ != value_t::integer)
+        {
+            this->throw_bad_cast("toml::value::as_integer_fmt()", value_t::integer);
+        }
+        return this->integer_.format;
+    }
+    floating_format_info& as_floating_fmt()
+    {
+        if(this->type_ != value_t::floating)
+        {
+            this->throw_bad_cast("toml::value::as_floating_fmt()", value_t::floating);
+        }
+        return this->floating_.format;
+    }
+    string_format_info& as_string_fmt()
+    {
+        if(this->type_ != value_t::string)
+        {
+            this->throw_bad_cast("toml::value::as_string_fmt()", value_t::string);
+        }
+        return this->string_.format;
+    }
+    offset_datetime_format_info& as_offset_datetime_fmt()
+    {
+        if(this->type_ != value_t::offset_datetime)
+        {
+            this->throw_bad_cast("toml::value::as_offset_datetime_fmt()", value_t::offset_datetime);
+        }
+        return this->offset_datetime_.format;
+    }
+    local_datetime_format_info& as_local_datetime_fmt()
+    {
+        if(this->type_ != value_t::local_datetime)
+        {
+            this->throw_bad_cast("toml::value::as_local_datetime_fmt()", value_t::local_datetime);
+        }
+        return this->local_datetime_.format;
+    }
+    local_date_format_info& as_local_date_fmt()
+    {
+        if(this->type_ != value_t::local_date)
+        {
+            this->throw_bad_cast("toml::value::as_local_date_fmt()", value_t::local_date);
+        }
+        return this->local_date_.format;
+    }
+    local_time_format_info& as_local_time_fmt()
+    {
+        if(this->type_ != value_t::local_time)
+        {
+            this->throw_bad_cast("toml::value::as_local_time_fmt()", value_t::local_time);
+        }
+        return this->local_time_.format;
+    }
+    array_format_info& as_array_fmt()
+    {
+        if(this->type_ != value_t::array)
+        {
+            this->throw_bad_cast("toml::value::as_array_fmt()", value_t::array);
+        }
+        return this->array_.format;
+    }
+    table_format_info& as_table_fmt()
+    {
+        if(this->type_ != value_t::table)
+        {
+            this->throw_bad_cast("toml::value::as_table_fmt()", value_t::table);
+        }
+        return this->table_.format;
+    }
+    // }}}
+
+    // table accessors ==================================================== {{{
+
+    value_type& at(const key_type& k)
+    {
+        if(!this->is_table())
+        {
+            this->throw_bad_cast("toml::value::at(key_type)", value_t::table);
+        }
+        auto& table = this->as_table(std::nothrow);
+        const auto found = table.find(k);
+        if(found == table.end())
+        {
+            this->throw_key_not_found_error("toml::value::at", k);
+        }
+        assert(found->first == k);
+        return found->second;
+    }
+    value_type const& at(const key_type& k) const
+    {
+        if(!this->is_table())
+        {
+            this->throw_bad_cast("toml::value::at(key_type)", value_t::table);
+        }
+        const auto& table = this->as_table(std::nothrow);
+        const auto found = table.find(k);
+        if(found == table.end())
+        {
+            this->throw_key_not_found_error("toml::value::at", k);
+        }
+        assert(found->first == k);
+        return found->second;
+    }
+    value_type& operator[](const key_type& k)
+    {
+        if(this->is_empty())
+        {
+            (*this) = table_type{};
+        }
+        else if( ! this->is_table()) // initialized, but not a table
+        {
+            this->throw_bad_cast("toml::value::operator[](key_type)", value_t::table);
+        }
+        return (this->as_table(std::nothrow))[k];
+    }
+    std::size_t count(const key_type& k) const
+    {
+        if(!this->is_table())
+        {
+            this->throw_bad_cast("toml::value::count(key_type)", value_t::table);
+        }
+        return this->as_table(std::nothrow).count(k);
+    }
+    bool contains(const key_type& k) const
+    {
+        if(!this->is_table())
+        {
+            this->throw_bad_cast("toml::value::contains(key_type)", value_t::table);
+        }
+        const auto& table = this->as_table(std::nothrow);
+        return table.find(k) != table.end();
+    }
+    // }}}
+
+    // array accessors ==================================================== {{{
+
+    value_type& at(const std::size_t idx)
+    {
+        if(!this->is_array())
+        {
+            this->throw_bad_cast("toml::value::at(idx)", value_t::array);
+        }
+        auto& ar = this->as_array(std::nothrow);
+
+        if(ar.size() <= idx)
+        {
+            std::ostringstream oss;
+            oss << "actual length (" << ar.size()
+                << ") is shorter than the specified index (" << idx << ").";
+            throw std::out_of_range(format_error(
+                "toml::value::at(idx): no element corresponding to the index",
+                this->location(), oss.str()
+                ));
+        }
+        return ar.at(idx);
+    }
+    value_type const& at(const std::size_t idx) const
+    {
+        if(!this->is_array())
+        {
+            this->throw_bad_cast("toml::value::at(idx)", value_t::array);
+        }
+        const auto& ar = this->as_array(std::nothrow);
+
+        if(ar.size() <= idx)
+        {
+            std::ostringstream oss;
+            oss << "actual length (" << ar.size()
+                << ") is shorter than the specified index (" << idx << ").";
+
+            throw std::out_of_range(format_error(
+                "toml::value::at(idx): no element corresponding to the index",
+                this->location(), oss.str()
+                ));
+        }
+        return ar.at(idx);
+    }
+
+    value_type&       operator[](const std::size_t idx) noexcept
+    {
+        // no check...
+        return this->as_array(std::nothrow)[idx];
+    }
+    value_type const& operator[](const std::size_t idx) const noexcept
+    {
+        // no check...
+        return this->as_array(std::nothrow)[idx];
+    }
+
+    void push_back(const value_type& x)
+    {
+        if(!this->is_array())
+        {
+            this->throw_bad_cast("toml::value::push_back(idx)", value_t::array);
+        }
+        this->as_array(std::nothrow).push_back(x);
+        return;
+    }
+    void push_back(value_type&& x)
+    {
+        if(!this->is_array())
+        {
+            this->throw_bad_cast("toml::value::push_back(idx)", value_t::array);
+        }
+        this->as_array(std::nothrow).push_back(std::move(x));
+        return;
+    }
+
+    template<typename ... Ts>
+    value_type& emplace_back(Ts&& ... args)
+    {
+        if(!this->is_array())
+        {
+            this->throw_bad_cast("toml::value::emplace_back(idx)", value_t::array);
+        }
+        auto& ar = this->as_array(std::nothrow);
+        ar.emplace_back(std::forward<Ts>(args) ...);
+        return ar.back();
+    }
+
+    std::size_t size() const
+    {
+        switch(this->type_)
+        {
+            case value_t::array:
+            {
+                return this->as_array(std::nothrow).size();
+            }
+            case value_t::table:
+            {
+                return this->as_table(std::nothrow).size();
+            }
+            case value_t::string:
+            {
+                return this->as_string(std::nothrow).size();
+            }
+            default:
+            {
+                throw type_error(format_error(
+                    "toml::value::size(): bad_cast to container types",
+                    this->location(),
+                    "the actual type is " + to_string(this->type_)
+                    ), this->location());
+            }
+        }
+    }
+
+    // }}}
+
+    source_location location() const
+    {
+        return source_location(this->region_);
+    }
+
+    comment_type const& comments() const noexcept {return this->comments_;}
+    comment_type&       comments()       noexcept {return this->comments_;}
+
+  private:
+
+    // private helper functions =========================================== {{{
+
+    void cleanup() noexcept
+    {
+        switch(this->type_)
+        {
+            case value_t::boolean         : { boolean_        .~boolean_storage         (); break; }
+            case value_t::integer         : { integer_        .~integer_storage         (); break; }
+            case value_t::floating        : { floating_       .~floating_storage        (); break; }
+            case value_t::string          : { string_         .~string_storage          (); break; }
+            case value_t::offset_datetime : { offset_datetime_.~offset_datetime_storage (); break; }
+            case value_t::local_datetime  : { local_datetime_ .~local_datetime_storage  (); break; }
+            case value_t::local_date      : { local_date_     .~local_date_storage      (); break; }
+            case value_t::local_time      : { local_time_     .~local_time_storage      (); break; }
+            case value_t::array           : { array_          .~array_storage           (); break; }
+            case value_t::table           : { table_          .~table_storage           (); break; }
+            default                       : { break; }
+        }
+        this->type_ = value_t::empty;
+        return;
+    }
+
+    template<typename T, typename U>
+    static void assigner(T& dst, U&& v)
+    {
+        const auto tmp = ::new(std::addressof(dst)) T(std::forward<U>(v));
+        assert(tmp == std::addressof(dst));
+        (void)tmp;
+    }
+
+    [[noreturn]]
+    void throw_bad_cast(const std::string& funcname, const value_t ty) const
+    {
+        throw type_error(format_error(detail::make_type_error(*this, funcname, ty)),
+                         this->location());
+    }
+
+    [[noreturn]]
+    void throw_key_not_found_error(const std::string& funcname, const key_type& key) const
+    {
+        throw std::out_of_range(format_error(
+                    detail::make_not_found_error(*this, funcname, key)));
+    }
+
+    template<typename TC>
+    friend void detail::change_region_of_value(basic_value<TC>&, const basic_value<TC>&);
+
+    template<typename TC>
+    friend class basic_value;
+
+    // }}}
+
+  private:
+
+    using boolean_storage         = detail::value_with_format<boolean_type,                boolean_format_info        >;
+    using integer_storage         = detail::value_with_format<integer_type,                integer_format_info        >;
+    using floating_storage        = detail::value_with_format<floating_type,               floating_format_info       >;
+    using string_storage          = detail::value_with_format<string_type,                 string_format_info         >;
+    using offset_datetime_storage = detail::value_with_format<offset_datetime_type,        offset_datetime_format_info>;
+    using local_datetime_storage  = detail::value_with_format<local_datetime_type,         local_datetime_format_info >;
+    using local_date_storage      = detail::value_with_format<local_date_type,             local_date_format_info     >;
+    using local_time_storage      = detail::value_with_format<local_time_type,             local_time_format_info     >;
+    using array_storage           = detail::value_with_format<detail::storage<array_type>, array_format_info          >;
+    using table_storage           = detail::value_with_format<detail::storage<table_type>, table_format_info          >;
+
+  private:
+
+    value_t type_;
+    union
+    {
+        char                    empty_; // the smallest type
+        boolean_storage         boolean_;
+        integer_storage         integer_;
+        floating_storage        floating_;
+        string_storage          string_;
+        offset_datetime_storage offset_datetime_;
+        local_datetime_storage  local_datetime_;
+        local_date_storage      local_date_;
+        local_time_storage      local_time_;
+        array_storage           array_;
+        table_storage           table_;
+    };
+    region_type  region_;
+    comment_type comments_;
+};
+
+template<typename TC>
+bool operator==(const basic_value<TC>& lhs, const basic_value<TC>& rhs)
+{
+    if(lhs.type()     != rhs.type())     {return false;}
+    if(lhs.comments() != rhs.comments()) {return false;}
+
+    switch(lhs.type())
+    {
+        case value_t::boolean  :
+        {
+            return lhs.as_boolean() == rhs.as_boolean();
+        }
+        case value_t::integer  :
+        {
+            return lhs.as_integer() == rhs.as_integer();
+        }
+        case value_t::floating :
+        {
+            return lhs.as_floating() == rhs.as_floating();
+        }
+        case value_t::string   :
+        {
+            return lhs.as_string() == rhs.as_string();
+        }
+        case value_t::offset_datetime:
+        {
+            return lhs.as_offset_datetime() == rhs.as_offset_datetime();
+        }
+        case value_t::local_datetime:
+        {
+            return lhs.as_local_datetime() == rhs.as_local_datetime();
+        }
+        case value_t::local_date:
+        {
+            return lhs.as_local_date() == rhs.as_local_date();
+        }
+        case value_t::local_time:
+        {
+            return lhs.as_local_time() == rhs.as_local_time();
+        }
+        case value_t::array    :
+        {
+            return lhs.as_array() == rhs.as_array();
+        }
+        case value_t::table    :
+        {
+            return lhs.as_table() == rhs.as_table();
+        }
+        case value_t::empty    : {return true; }
+        default:                 {return false;}
+    }
+}
+
+template<typename TC>
+bool operator!=(const basic_value<TC>& lhs, const basic_value<TC>& rhs)
+{
+    return !(lhs == rhs);
+}
+
+template<typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_comparable<typename basic_value<TC>::array_type>,
+    detail::is_comparable<typename basic_value<TC>::table_type>
+    >::value, bool>
+operator<(const basic_value<TC>& lhs, const basic_value<TC>& rhs)
+{
+    if(lhs.type() != rhs.type())
+    {
+        return (lhs.type() < rhs.type());
+    }
+    switch(lhs.type())
+    {
+        case value_t::boolean  :
+        {
+            return lhs.as_boolean() <  rhs.as_boolean() ||
+                  (lhs.as_boolean() == rhs.as_boolean() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::integer  :
+        {
+            return lhs.as_integer() <  rhs.as_integer() ||
+                  (lhs.as_integer() == rhs.as_integer() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::floating :
+        {
+            return lhs.as_floating() <  rhs.as_floating() ||
+                  (lhs.as_floating() == rhs.as_floating() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::string   :
+        {
+            return lhs.as_string() <  rhs.as_string() ||
+                  (lhs.as_string() == rhs.as_string() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::offset_datetime:
+        {
+            return lhs.as_offset_datetime() <  rhs.as_offset_datetime() ||
+                  (lhs.as_offset_datetime() == rhs.as_offset_datetime() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::local_datetime:
+        {
+            return lhs.as_local_datetime() <  rhs.as_local_datetime() ||
+                  (lhs.as_local_datetime() == rhs.as_local_datetime() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::local_date:
+        {
+            return lhs.as_local_date() <  rhs.as_local_date() ||
+                  (lhs.as_local_date() == rhs.as_local_date() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::local_time:
+        {
+            return lhs.as_local_time() <  rhs.as_local_time() ||
+                  (lhs.as_local_time() == rhs.as_local_time() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::array    :
+        {
+            return lhs.as_array() <  rhs.as_array() ||
+                  (lhs.as_array() == rhs.as_array() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::table    :
+        {
+            return lhs.as_table() <  rhs.as_table() ||
+                  (lhs.as_table() == rhs.as_table() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::empty    :
+        {
+            return lhs.comments() < rhs.comments();
+        }
+        default:
+        {
+            return lhs.comments() < rhs.comments();
+        }
+    }
+}
+
+template<typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_comparable<typename basic_value<TC>::array_type>,
+    detail::is_comparable<typename basic_value<TC>::table_type>
+    >::value, bool>
+operator<=(const basic_value<TC>& lhs, const basic_value<TC>& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+template<typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_comparable<typename basic_value<TC>::array_type>,
+    detail::is_comparable<typename basic_value<TC>::table_type>
+    >::value, bool>
+operator>(const basic_value<TC>& lhs, const basic_value<TC>& rhs)
+{
+    return !(lhs <= rhs);
+}
+template<typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_comparable<typename basic_value<TC>::array_type>,
+    detail::is_comparable<typename basic_value<TC>::table_type>
+    >::value, bool>
+operator>=(const basic_value<TC>& lhs, const basic_value<TC>& rhs)
+{
+    return !(lhs < rhs);
+}
+
+// error_info helper
+namespace detail
+{
+template<typename TC, typename ... Ts>
+error_info make_error_info_rec(error_info e,
+        const basic_value<TC>& v, std::string msg, Ts&& ... tail)
+{
+    return make_error_info_rec(std::move(e), v.location(), std::move(msg), std::forward<Ts>(tail)...);
+}
+} // detail
+
+template<typename TC, typename ... Ts>
+error_info make_error_info(
+        std::string title, const basic_value<TC>& v, std::string msg, Ts&& ... tail)
+{
+    return make_error_info(std::move(title),
+            v.location(), std::move(msg), std::forward<Ts>(tail)...);
+}
+template<typename TC, typename ... Ts>
+std::string format_error(std::string title,
+        const basic_value<TC>& v, std::string msg, Ts&& ... tail)
+{
+    return format_error(std::move(title),
+            v.location(), std::move(msg), std::forward<Ts>(tail)...);
+}
+
+namespace detail
+{
+
+template<typename TC>
+error_info make_type_error(const basic_value<TC>& v, const std::string& fname, const value_t ty)
+{
+    return make_error_info(fname + ": bad_cast to " + to_string(ty),
+        v.location(), "the actual type is " + to_string(v.type()));
+}
+template<typename TC>
+error_info make_not_found_error(const basic_value<TC>& v, const std::string& fname, const typename basic_value<TC>::key_type& key)
+{
+    const auto loc = v.location();
+    const std::string title = fname + ": key \"" + string_conv<std::string>(key) + "\" not found";
+
+    std::vector<std::pair<source_location, std::string>> locs;
+    if( ! loc.is_ok())
+    {
+        return error_info(title, locs);
+    }
+
+    if(loc.first_line_number() == 1 && loc.first_column_number() == 1 && loc.length() == 1)
+    {
+        // The top-level table has its region at the 0th character of the file.
+        // That means that, in the case when a key is not found in the top-level
+        // table, the error message points to the first character. If the file has
+        // the first table at the first line, the error message would be like this.
+        // ```console
+        // [error] key "a" not found
+        //  --> example.toml
+        //    |
+        //  1 | [table]
+        //    | ^------ in this table
+        // ```
+        // It actually points to the top-level table at the first character, not
+        // `[table]`. But it is too confusing. To avoid the confusion, the error
+        // message should explicitly say "key not found in the top-level table".
+        locs.emplace_back(v.location(), "at the top-level table");
+    }
+    else
+    {
+        locs.emplace_back(v.location(), "in this table");
+    }
+    return error_info(title, locs);
+}
+
+#define TOML11_DETAIL_GENERATE_COMPTIME_GETTER(ty)                              \
+    template<typename TC>                                                       \
+    struct getter<TC, value_t::ty>                                              \
+    {                                                                           \
+        using value_type = basic_value<TC>;                                     \
+        using result_type = enum_to_type_t<value_t::ty, value_type>;            \
+        using format_type = enum_to_fmt_type_t<value_t::ty>;                    \
+                                                                                \
+        static result_type&       get(value_type& v)                            \
+        {                                                                       \
+            return v.as_ ## ty();                                               \
+        }                                                                       \
+        static result_type const& get(const value_type& v)                      \
+        {                                                                       \
+            return v.as_ ## ty();                                               \
+        }                                                                       \
+                                                                                \
+        static result_type&       get_nothrow(value_type& v) noexcept           \
+        {                                                                       \
+            return v.as_ ## ty(std::nothrow);                                   \
+        }                                                                       \
+        static result_type const& get_nothrow(const value_type& v) noexcept     \
+        {                                                                       \
+            return v.as_ ## ty(std::nothrow);                                   \
+        }                                                                       \
+                                                                                \
+        static format_type&       get_fmt(value_type& v)                        \
+        {                                                                       \
+            return v.as_ ## ty ## _fmt();                                       \
+        }                                                                       \
+        static format_type const& get_fmt(const value_type& v)                  \
+        {                                                                       \
+            return v.as_ ## ty ## _fmt();                                       \
+        }                                                                       \
+                                                                                \
+        static format_type&       get_fmt_nothrow(value_type& v) noexcept       \
+        {                                                                       \
+            return v.as_ ## ty ## _fmt(std::nothrow);                           \
+        }                                                                       \
+        static format_type const& get_fmt_nothrow(const value_type& v) noexcept \
+        {                                                                       \
+            return v.as_ ## ty ## _fmt(std::nothrow);                           \
+        }                                                                       \
+    };
+
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(boolean        )
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(integer        )
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(floating       )
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(string         )
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(offset_datetime)
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(local_datetime )
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(local_date     )
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(local_time     )
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(array          )
+TOML11_DETAIL_GENERATE_COMPTIME_GETTER(table          )
+
+#undef TOML11_DETAIL_GENERATE_COMPTIME_GETTER
+
+template<typename TC>
+void change_region_of_value(basic_value<TC>& dst, const basic_value<TC>& src)
+{
+    dst.region_ = std::move(src.region_);
+    return;
+}
+
+} // namespace detail
+} // namespace toml
+#endif // TOML11_VALUE_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/value_t.hpp b/src/frontend/qt_sdl/toml/toml11/value_t.hpp
new file mode 100644
index 00000000..ed0fbe93
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/value_t.hpp
@@ -0,0 +1,10 @@
+#ifndef TOML11_VALUE_T_HPP
+#define TOML11_VALUE_T_HPP
+
+#include "fwd/value_t_fwd.hpp" // IWYU pragma: export
+
+#if ! defined(TOML11_COMPILE_SOURCES)
+#include "impl/value_t_impl.hpp" // IWYU pragma: export
+#endif
+
+#endif // TOML11_VALUE_T_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/version.hpp b/src/frontend/qt_sdl/toml/toml11/version.hpp
new file mode 100644
index 00000000..f95b3cf5
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/version.hpp
@@ -0,0 +1,121 @@
+#ifndef TOML11_VERSION_HPP
+#define TOML11_VERSION_HPP
+
+#define TOML11_VERSION_MAJOR 4
+#define TOML11_VERSION_MINOR 2
+#define TOML11_VERSION_PATCH 0
+
+#ifndef __cplusplus
+#    error "__cplusplus is not defined"
+#endif
+
+// Since MSVC does not define `__cplusplus` correctly unless you pass
+// `/Zc:__cplusplus` when compiling, the workaround macros are added.
+//
+// The value of `__cplusplus` macro is defined in the C++ standard spec, but
+// MSVC ignores the value, maybe because of backward compatibility. Instead,
+// MSVC defines _MSVC_LANG that has the same value as __cplusplus defined in
+// the C++ standard. So we check if _MSVC_LANG is defined before using `__cplusplus`.
+//
+// FYI: https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-170
+//      https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-170
+//
+
+#if defined(_MSVC_LANG) && defined(_MSC_VER) && 190024210 <= _MSC_FULL_VER
+#  define TOML11_CPLUSPLUS_STANDARD_VERSION _MSVC_LANG
+#else
+#  define TOML11_CPLUSPLUS_STANDARD_VERSION __cplusplus
+#endif
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION < 201103L
+#    error "toml11 requires C++11 or later."
+#endif
+
+#if ! defined(__has_include)
+#  define __has_include(x) 0
+#endif
+
+#if ! defined(__has_cpp_attribute)
+#  define __has_cpp_attribute(x) 0
+#endif
+
+#if ! defined(__has_builtin)
+#  define __has_builtin(x) 0
+#endif
+
+// hard to remember
+
+#ifndef TOML11_CXX14_VALUE
+#define TOML11_CXX14_VALUE 201402L
+#endif//TOML11_CXX14_VALUE
+
+#ifndef TOML11_CXX17_VALUE
+#define TOML11_CXX17_VALUE 201703L
+#endif//TOML11_CXX17_VALUE
+
+#ifndef TOML11_CXX20_VALUE
+#define TOML11_CXX20_VALUE 202002L
+#endif//TOML11_CXX20_VALUE
+
+#if defined(__cpp_char8_t)
+#  if __cpp_char8_t >= 201811L
+#    define TOML11_HAS_CHAR8_T 1
+#  endif
+#endif
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX17_VALUE
+#  if __has_include(<string_view>)
+#    define TOML11_HAS_STRING_VIEW 1
+#  endif
+#endif
+
+#ifndef TOML11_DISABLE_STD_FILESYSTEM
+#  if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX17_VALUE
+#    if __has_include(<filesystem>)
+#      define TOML11_HAS_FILESYSTEM 1
+#    endif
+#  endif
+#endif
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= TOML11_CXX17_VALUE
+#  if __has_include(<optional>)
+#    define TOML11_HAS_OPTIONAL 1
+#  endif
+#endif
+
+#if defined(TOML11_COMPILE_SOURCES)
+#  define TOML11_INLINE
+#else
+#  define TOML11_INLINE inline
+#endif
+
+namespace toml
+{
+
+inline const char* license_notice() noexcept
+{
+    return R"(The MIT License (MIT)
+
+Copyright (c) 2017-now Toru Niina
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.)";
+}
+
+} // toml
+#endif // TOML11_VERSION_HPP
diff --git a/src/frontend/qt_sdl/toml/toml11/visit.hpp b/src/frontend/qt_sdl/toml/toml11/visit.hpp
new file mode 100644
index 00000000..d97344b3
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml11/visit.hpp
@@ -0,0 +1,81 @@
+#ifndef TOML11_VISIT_HPP
+#define TOML11_VISIT_HPP
+
+#include "exception.hpp"
+#include "traits.hpp"
+#include "value.hpp"
+
+namespace toml
+{
+
+template<typename Visitor, typename TC>
+cxx::return_type_of_t<Visitor, const typename basic_value<TC>::boolean_type&>
+visit(Visitor&& visitor, const basic_value<TC>& v)
+{
+    switch(v.type())
+    {
+        case value_t::boolean        : {return visitor(v.as_boolean        ());}
+        case value_t::integer        : {return visitor(v.as_integer        ());}
+        case value_t::floating       : {return visitor(v.as_floating       ());}
+        case value_t::string         : {return visitor(v.as_string         ());}
+        case value_t::offset_datetime: {return visitor(v.as_offset_datetime());}
+        case value_t::local_datetime : {return visitor(v.as_local_datetime ());}
+        case value_t::local_date     : {return visitor(v.as_local_date     ());}
+        case value_t::local_time     : {return visitor(v.as_local_time     ());}
+        case value_t::array          : {return visitor(v.as_array          ());}
+        case value_t::table          : {return visitor(v.as_table          ());}
+        case value_t::empty          : break;
+        default: break;
+    }
+    throw type_error(format_error("[error] toml::visit: toml::basic_value "
+            "does not have any valid type.", v.location(), "here"), v.location());
+}
+
+template<typename Visitor, typename TC>
+cxx::return_type_of_t<Visitor, typename basic_value<TC>::boolean_type&>
+visit(Visitor&& visitor, basic_value<TC>& v)
+{
+    switch(v.type())
+    {
+        case value_t::boolean        : {return visitor(v.as_boolean        ());}
+        case value_t::integer        : {return visitor(v.as_integer        ());}
+        case value_t::floating       : {return visitor(v.as_floating       ());}
+        case value_t::string         : {return visitor(v.as_string         ());}
+        case value_t::offset_datetime: {return visitor(v.as_offset_datetime());}
+        case value_t::local_datetime : {return visitor(v.as_local_datetime ());}
+        case value_t::local_date     : {return visitor(v.as_local_date     ());}
+        case value_t::local_time     : {return visitor(v.as_local_time     ());}
+        case value_t::array          : {return visitor(v.as_array          ());}
+        case value_t::table          : {return visitor(v.as_table          ());}
+        case value_t::empty          : break;
+        default: break;
+    }
+    throw type_error(format_error("[error] toml::visit: toml::basic_value "
+            "does not have any valid type.", v.location(), "here"), v.location());
+}
+
+template<typename Visitor, typename TC>
+cxx::return_type_of_t<Visitor, typename basic_value<TC>::boolean_type&&>
+visit(Visitor&& visitor, basic_value<TC>&& v)
+{
+    switch(v.type())
+    {
+        case value_t::boolean        : {return visitor(std::move(v.as_boolean        ()));}
+        case value_t::integer        : {return visitor(std::move(v.as_integer        ()));}
+        case value_t::floating       : {return visitor(std::move(v.as_floating       ()));}
+        case value_t::string         : {return visitor(std::move(v.as_string         ()));}
+        case value_t::offset_datetime: {return visitor(std::move(v.as_offset_datetime()));}
+        case value_t::local_datetime : {return visitor(std::move(v.as_local_datetime ()));}
+        case value_t::local_date     : {return visitor(std::move(v.as_local_date     ()));}
+        case value_t::local_time     : {return visitor(std::move(v.as_local_time     ()));}
+        case value_t::array          : {return visitor(std::move(v.as_array          ()));}
+        case value_t::table          : {return visitor(std::move(v.as_table          ()));}
+        case value_t::empty          : break;
+        default: break;
+    }
+    throw type_error(format_error("[error] toml::visit: toml::basic_value "
+            "does not have any valid type.", v.location(), "here"), v.location());
+}
+
+} // toml
+#endif // TOML11_VISIT_HPP
diff --git a/src/frontend/qt_sdl/toml/toml_fwd.hpp b/src/frontend/qt_sdl/toml/toml_fwd.hpp
new file mode 100644
index 00000000..3ba77ff4
--- /dev/null
+++ b/src/frontend/qt_sdl/toml/toml_fwd.hpp
@@ -0,0 +1,88 @@
+#ifndef TOML11_TOML_FWD_HPP
+#define TOML11_TOML_FWD_HPP
+
+// IWYU pragma: begin_exports
+#include "toml11/version.hpp"
+#include "toml11/compat.hpp"
+#include "toml11/conversion.hpp"
+#include "toml11/from.hpp"
+#include "toml11/into.hpp"
+// IWYU pragma: end_exports
+
+#include <cstdint>
+
+#ifdef TOML11_COLORIZE_ERROR_MESSAGE
+#define TOML11_ERROR_MESSAGE_COLORIZED true
+#else
+#define TOML11_ERROR_MESSAGE_COLORIZED false
+#endif
+
+namespace toml
+{
+class discard_comments;
+class preserve_comments;
+
+enum class month_t : std::uint8_t;
+struct local_date;
+struct local_time;
+struct time_offset;
+struct local_datetime;
+struct offset_datetime;
+
+struct error_info;
+
+struct exception;
+
+enum class indent_char : std::uint8_t;
+enum class integer_format : std::uint8_t;
+enum class floating_format : std::uint8_t;
+enum class string_format : std::uint8_t;
+enum class datetime_delimiter_kind : std::uint8_t;
+enum class array_format : std::uint8_t;
+enum class table_format : std::uint8_t;
+
+struct boolean_format_info;
+struct integer_format_info;
+struct floating_format_info;
+struct string_format_info;
+struct offset_datetime_format_info;
+struct local_datetime_format_info;
+struct local_date_format_info;
+struct local_time_format_info;
+struct array_format_info;
+struct table_format_info;
+
+template<typename Key, typename Val, typename Cmp, typename Allocator>
+class ordered_map;
+
+struct syntax_error;
+struct file_io_error;
+
+struct bad_result_access;
+template<typename T>
+struct success;
+template<typename T>
+struct failure;
+template<typename T, typename E>
+struct result;
+
+struct source_location;
+
+struct semantic_version;
+struct spec;
+
+
+template<typename TypeConfig>
+class basic_value;
+struct type_error;
+
+struct type_config;
+using value = basic_value<type_config>;
+
+struct ordered_type_config;
+using ordered_value = basic_value<ordered_type_config>;
+
+enum class value_t : std::uint8_t;
+
+} // toml
+#endif// TOML11_TOML_HPP