MAYBE We are left with following problem, upon which TcT provides the certificate MAYBE. Strict Trs: { ack(0(), y) -> s(y) , ack(s(x), 0()) -> ack(x, s(0())) , ack(s(x), s(y)) -> ack(x, ack(s(x), y)) } Obligation: innermost runtime complexity Answer: MAYBE The input cannot be shown compatible Arrrr..