WString Filter(const wchar *s, int (*filter)(int))
{
	WString result;
	while(*s) {
		int c = (*filter)((char)*s++);
//                               ^^^^^^^^^ bug, should be wchar
		if(c) result.Cat(c);
	}
	return result;
}

String Filter(const char *s, int (*filter)(int))
{
	String result;
	while(*s) {
		int c = (*filter)((byte)*s++);
//                               ^^^^^^^ problem when s is UTF-8
		if(c) result.Cat(c);
	}
	return result;
}