The iswblank function is the wide-character equivalent of the
isblank function. It tests whether wc is a wide character
belonging to the wide character class "blank".
The wide character class "blank" is a subclass of the wide character class
"space".
Being a subclass of the wide character class "space", the wide character class
"blank" is disjoint from the wide character class "graph" and therefore also
disjoint from its subclasses "alnum", "alpha", "upper", "lower", "digit",
"xdigit", "punct".
The wide character class "blank" always contains at least the space character
and the control character '\\t'.