rev ls r1 ls empty r1 empty a a r1 cons x k a r1 k cons x a 2.2 rev ls r1 ls empty r1 cons x k a r1 k cons x a true rev ls r1 ls empty false r1 cons x k a r1 k cons x a true rev 1 0 rev 1 0 r1 2 1 0 r1 2 0 cons 2 2 1 empty 0 0 NaTT certifiable-1.6