MAYBE We are left with following problem, upon which TcT provides the certificate MAYBE. Strict Trs: { first(0(), X) -> nil() , first(s(X), cons(Y, Z)) -> cons(Y, first(X, Z)) , from(X) -> cons(X, from(s(X))) } Obligation: innermost runtime complexity Answer: MAYBE The input cannot be shown compatible Arrrr..