MAYBE We are left with following problem, upon which TcT provides the certificate MAYBE. Strict Trs: { from(X) -> cons(X, n__from(s(X))) , from(X) -> n__from(X) , first(X1, X2) -> n__first(X1, X2) , first(s(X), cons(Y, Z)) -> cons(Y, n__first(X, activate(Z))) , first(0(), Z) -> nil() , activate(X) -> X , activate(n__from(X)) -> from(X) , activate(n__first(X1, X2)) -> first(X1, X2) , sel(s(X), cons(Y, Z)) -> sel(X, activate(Z)) , sel(0(), cons(X, Z)) -> X } Obligation: innermost runtime complexity Answer: MAYBE The input cannot be shown compatible Arrrr..