TRS:
 {                   from(X) -> cons(X, n__from(s(X))),
               first(0(), Z) -> nil(),
     first(s(X), cons(Y, Z)) -> cons(Y, n__first(X, activate(Z))),
        sel(0(), cons(X, Z)) -> X,
       sel(s(X), cons(Y, Z)) -> sel(X, activate(Z)),
                     from(X) -> n__from(X),
               first(X1, X2) -> n__first(X1, X2),
        activate(n__from(X)) -> from(X),
  activate(n__first(X1, X2)) -> first(X1, X2),
                 activate(X) -> X}
 Fail