is_emptyniltrueis_emptyconsxlfalsehdconsxlxtlconsxllappendl1l2ifappendl1l2l1ifappendl1l2nill2ifappendl1l2consxlconsxappendll2appendl1l2ifappendl1l2l1ifappendl1l2consxlappendll2true1ifappend37102130append281020cons201821appendl1l2ifappendl1l2l1appendl1l2ifappendl1l2l1false