(VAR x y )
(RULES 
        exp(x, 0) -> s(0)
        exp(x, s(y)) -> *(x, exp(x, y))
        *(0, y) -> 0
        *(s(x), y) -> +(y, *(x, y))
        -(0, y) -> 0
        -(x, 0) -> x
        -(s(x), s(y)) -> -(x, y)
        
)