TRS:
 {      numbers() -> d(0()),
             d(x) -> if(le(x, nr()), x),
    if(true(), x) -> cons(x, d(s(x))),
   if(false(), x) -> nil(),
       le(0(), y) -> true(),
    le(s(x), 0()) -> false(),
   le(s(x), s(y)) -> le(x, y),
             nr() -> ack(s(s(s(s(s(s(0())))))), 0()),
      ack(0(), x) -> s(x),
   ack(s(x), 0()) -> ack(x, s(0())),
  ack(s(x), s(y)) -> ack(x, ack(s(x), y))}
 Fail