sub 0 0 0 sub s x 0 s x sub 0 s x 0 sub s x s y sub x y zero nil zero2 0 nil zero cons x xs zero2 sub x x cons x xs zero2 0 nil nil zero2 0 cons x xs cons sub x x zero xs zero2 s y nil zero nil zero2 s y cons x xs zero cons x xs 2.2 zero cons x xs sub x x zero2 0 cons x xs sub x x zero2 0 cons x xs zero xs zero nil zero2 0 nil zero2 s y nil zero nil zero cons x xs zero2 sub x x cons x xs sub s x s y sub x y zero2 s y cons x xs zero cons x xs true zero2 s y nil zero nil false zero2 s y cons x xs zero cons x xs zero cons x xs zero2 sub x x cons x xs zero2 0 cons x xs zero xs true zero 1 0 s 1 1 2 sub 2 1 sub 2 0 zero2 2 0 0 0 2 nil 0 0 cons 2 2 1 zero 1 1 0 zero2 2 2 0 zero cons x xs zero2 sub x x cons x xs zero2 s y cons x xs zero cons x xs zero2 s y cons x xs zero cons x xs zero cons x xs zero2 sub x x cons x xs true Failed! NaTT certifiable-1.6