MAYBE We are left with following problem, upon which TcT provides the certificate MAYBE. Strict Trs: { app(nil(), xs) -> nil() , app(cons(x, xs), ys) -> cons(x, app(xs, ys)) , rev(nil()) -> nil() , rev(cons(x, xs)) -> append(xs, rev(cons(x, nil()))) } Obligation: innermost runtime complexity Answer: MAYBE The input cannot be shown compatible Arrrr..