MAYBE
Trs:
 {           g(0()) -> 0(),
            g(s(x)) -> minus(s(x), f(g(x))),
             f(0()) -> s(0()),
            f(s(x)) -> minus(s(x), g(f(x))),
  minus(s(x), s(y)) -> minus(x, y),
      minus(x, 0()) -> x}
 Comment:
  We consider a duplicating trs.
  FAIL:
   Open