TRS:
 {                 f(X) -> cons(X, n__f(n__g(X))),
                 g(0()) -> s(0()),
                g(s(X)) -> s(s(g(X))),
   sel(0(), cons(X, Y)) -> X,
  sel(s(X), cons(Y, Z)) -> sel(X, activate(Z)),
                   f(X) -> n__f(X),
                   g(X) -> n__g(X),
      activate(n__f(X)) -> f(activate(X)),
      activate(n__g(X)) -> g(activate(X)),
            activate(X) -> X}
 Fail