SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
Ensures that the destruction logic is kept local to the translation unit
(making it nicer when it comes to forward declaring non-trivial types).
It also doesn't really do much to define it in the header.
Given we're simply storing the std::string into a deque. We can emplace
it and move it. Completely avoiding copies with the current usage of the
function.
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