f true xs f isList xs append cons a nil xs isList nil true isList cons x xs isList xs append nil ys ys append cons x xs ys cons x append xs ys 2.2 isList cons x xs isList xs f true xs append cons a nil xs f true xs f isList xs append cons a nil xs append cons x xs ys append xs ys f true xs isList xs true f true xs f isList xs append cons a nil xs true Failed! NaTT certifiable-1.6