(VAR x y )
(RULES 
        ack(0, y) -> s(y)
        ack(s(x), 0) -> ack(x, s(0))
        ack(s(x), s(y)) -> ack(x, ack(s(x), y))
        f(s(x), y) -> f(x, s(x))
        f(x, s(y)) -> f(y, x)
        f(x, y) -> ack(x, y)
        ack(s(x), y) -> f(x, x)
        
)