MAYBE
Trs:
 {             fst(0(), Z) -> nil(),
     fst(s(X), cons(Y, Z)) -> cons(Y, n__fst(activate(X), activate(Z))),
               fst(X1, X2) -> n__fst(X1, X2),
                   from(X) -> cons(X, n__from(n__s(X))),
                   from(X) -> n__from(X),
                      s(X) -> n__s(X),
               add(0(), X) -> X,
              add(s(X), Y) -> s(n__add(activate(X), Y)),
               add(X1, X2) -> n__add(X1, X2),
                len(nil()) -> 0(),
           len(cons(X, Z)) -> s(n__len(activate(Z))),
                    len(X) -> n__len(X),
  activate(n__fst(X1, X2)) -> fst(activate(X1), activate(X2)),
      activate(n__from(X)) -> from(activate(X)),
         activate(n__s(X)) -> s(X),
  activate(n__add(X1, X2)) -> add(activate(X1), activate(X2)),
       activate(n__len(X)) -> len(activate(X)),
               activate(X) -> X}
 Comment:
  We consider a duplicating trs.
  FAIL:
   Open