MAYBE We are left with following problem, upon which TcT provides the certificate MAYBE. Strict Trs: { fib(N) -> sel(N, fib1(s(0()), s(0()))) , sel(s(N), cons(X, XS)) -> sel(N, activate(XS)) , sel(0(), cons(X, XS)) -> X , fib1(X, Y) -> cons(X, n__fib1(Y, add(X, Y))) , fib1(X1, X2) -> n__fib1(X1, X2) , add(s(X), Y) -> s(add(X, Y)) , add(0(), X) -> X , activate(X) -> X , activate(n__fib1(X1, X2)) -> fib1(X1, X2) } Obligation: innermost runtime complexity Answer: MAYBE The input cannot be shown compatible Arrrr..