length nil 0 length cons x l s length l lt x 0 false lt 0 s y true lt s x s y lt x y head cons x l x head nil undefined tail nil nil tail cons x l l reverse l rev 0 l nil l rev x l accu orig if lt x length orig x l accu orig if true x l accu orig rev s x tail l cons head l accu orig if false x l accu orig accu length 1 nil 0 0 0 cons 2 s 1 lt 2 false 0 true 0 head 1 undefined 0 tail 1 reverse 1 rev 4 if 5 FULL ./TRS/SchneiderKamp/trs/thiemann38.trs