class.c:415
VALUE
rb_mod_include_p(mod, mod2)
    VALUE mod;
    VALUE mod2;
{
    VALUE p;

    Check_Type(mod2, T_MODULE);
    for (p = RCLASS(mod)->super; p; p = RCLASS(p)->super) {
	if (BUILTIN_TYPE(p) == T_ICLASS) {
	    if (RBASIC(p)->klass == mod2) return Qtrue;
	}
    }
    return Qfalse;
}
