This is more idiomatic than what we had before.
We keep the trait bounds for the inherent impl, because the new method
can use them to guide inference of unannotated closure arguments.
The files in this directory make up the content of the course. The files here
can include third-party content from ../third_party/ as well.
When we publish a translation of the course, we git restore the src/ and
third_party/ directories at the repository root back to the date listed in the
POT-Creation-Date header of the translation. It is crucial, that all
translatable content lives in those two directories. The other files (such as
book.toml and theme/) are not restored and we always use the latest version
of them.
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