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