The towlower() function has as a domain a type wint_t, the value of which must be a character representable as a wchar_t, and must be a wide-character
code corresponding to a valid character in the current locale or the value of WEOF. If the argument has any other value, the argument is returned unchanged. If the argument of towlower() represents an upper-case wide-character code, and there exists a corresponding lower-case wide-character code (as defined by character type information in the program locale category LC_CTYPE), the result is the corresponding lower-case wide-character code. All other arguments in the domain are returned unchanged.