MAYBE We are left with following problem, upon which TcT provides the certificate MAYBE. Strict Trs: { U11(tt(), M, N) -> U12(tt(), activate(M), activate(N)) , U12(tt(), M, N) -> s(plus(activate(N), activate(M))) , activate(X) -> X , plus(N, s(M)) -> U11(tt(), M, N) , plus(N, 0()) -> N , U21(tt(), M, N) -> U22(tt(), activate(M), activate(N)) , U22(tt(), M, N) -> plus(x(activate(N), activate(M)), activate(N)) , x(N, s(M)) -> U21(tt(), M, N) , x(N, 0()) -> 0() } Obligation: innermost runtime complexity Answer: MAYBE The input cannot be shown compatible Arrrr..