f a empty g a empty f a cons x k f cons x a k g empty d d g cons x k d g k cons x d 2.2 f a empty g a empty g cons x k d g k cons x d f a cons x k f cons x a k true f a cons x k f cons x a k true f 2 0 f 2 2 0 g 2 0 cons 2 2 1 empty 0 0 g 2 0 f a empty g a empty false g cons x k d g k cons x d true f 2 0 f 2 0 g 2 1 0 cons 2 2 1 empty 0 0 g 2 0 NaTT certifiable-1.6