is_empty nil true is_empty cons x l false hd cons x l x tl cons x l l append l1 l2 ifappend l1 l2 l1 ifappend l1 l2 nil l2 ifappend l1 l2 cons x l cons x append l l2 is_empty 1 nil 0 true 0 cons 2 false 0 hd 1 tl 1 append 2 ifappend 3 FULL ./TRS/Cime/append.trs