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String upper and lower conversion are locale dependent and
implemented by the underlying libc, whereas Unicode
upper/lower conversion is not and only depends on the
Unicode character database.
OTOH, there are special cases where the standard Unicode
upper/lower mapping is no what you might expect, since the
database only provides a single mapping and is not context
aware.
There's nothing we can do if the libc is broken in some
respect. As for the extended case mapping support in
Unicode: patches are welcome.