The old implementation turns a single optional argument into two
arguments: the optional value and a boolean to indicate whether the
optional value is supplied.
The new expansion uses `unsafe-undefined` in place of not-supplied
arguments, in the general case. If the default-value expression is
simple enough, however, it is copied to call sites that would
otherwise supply `unsafe-undefined`. In the common case where the
default value is `#f`, for example, no run-time test is needed in the
core implementation function to check whether the default is supplied,
because a `#f` will be filled in for callers.
The performance improvement is tiny to non-existent for realistic
programs, but the simpler and reduced generated code may help in the
long run.
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