isEmpty empty true isEmpty node l x r false left empty empty left node l x r l right empty empty right node l x r r elem node l x r x append nil x cons x nil append cons y ys x cons y append ys x listify n xs if isEmpty n isEmpty left n right n node left left n elem left n node right left n elem n right n xs append xs n if true b n m xs ys xs if false false n m xs ys listify m xs if false true n m xs ys listify n ys toList n listify n nil isEmpty 1 empty 0 true 0 node 3 false 0 left 1 right 1 elem 1 append 2 nil 0 cons 2 y 0 listify 2 if 6 toList 1 FULL ./TRS/secret06/aprove/toList.trs