is_emptyniltrueis_emptyconsxlfalsehdconsxlxtlconsxllappendl1l2ifappendl1l2l1ifappendl1l2nill2ifappendl1l2consxlconsxappendll2appendl1l2ifappendl1l2l1ifappendl1l2consxlappendll2true1append2111ifappend3113cons211112is_empty1111nil01true01false01hd111tl111append21112ifappend31213appendl1l2ifappendl1l2l1appendl1l2ifappendl1l2l1false