The wctrans_t type represents a mapping which can map a wide character to
another wide character. Its nature is implementation dependent, but the
special value (wctrans_t)0 denotes an invalid mapping. Nonzero
wctrans_t values can be passed to the towctrans function to
actually perform the wide character mapping.
The wctrans function returns a mapping, given by its name. The set of
valid names depends on the LC_CTYPE category of the current locale, but the
following names are valid in all locales.
"tolower" - realizes the tolower(3) mapping
"toupper" - realizes the toupper(3) mapping