TRS:
 {plus(s(s(x)), y) -> s(plus(x, s(y))),
  plus(x, s(s(y))) -> s(plus(s(x), y)),
   plus(s(0()), y) -> s(y),
      plus(0(), y) -> y,
       ack(0(), y) -> s(y),
    ack(s(x), 0()) -> ack(x, s(0())),
   ack(s(x), s(y)) -> ack(x, plus(y, ack(s(x), y)))}
 Fail