class.c:131
void
rb_singleton_class_attached(klass, obj)
    VALUE klass, obj;
{
    if (FL_TEST(klass, FL_SINGLETON)) {
	if (!RCLASS(klass)->iv_tbl) {
	    RCLASS(klass)->iv_tbl = st_init_numtable();
	}
	st_insert(RCLASS(klass)->iv_tbl, rb_intern("__attached__"), obj);
    }
}
