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 2.2 rev x l accu orig if lt x length orig x l accu orig if true x l accu orig tail l lt s x s y lt x y if true x l accu orig head l if true x l accu orig rev s x tail l cons head l accu orig rev x l accu orig lt x length orig length cons x l length l reverse l rev 0 l nil l rev x l accu orig length orig true reverse l rev 0 l nil l false if true x l accu orig rev s x tail l cons head l accu orig rev x l accu orig if lt x length orig x l accu orig true Failed! NaTT certifiable-1.6