f empty l l f cons x k l g k l cons x k g a b c f a cons b c 2.2 g a b c f a cons b c f cons x k l g k l cons x k true f cons x k l g k l cons x k g a b c f a cons b c true f 2 0 f 2 1 0 g 3 1 1 cons 2 2 2 empty 0 0 g 3 0 NaTT certifiable-1.6