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