(VAR x y )
(RULES 
        O(0) -> 0
        +(0, x) -> x
        +(x, 0) -> x
        +(O(x), O(y)) -> O(+(x, y))
        +(O(x), I(y)) -> I(+(x, y))
        +(I(x), O(y)) -> I(+(x, y))
        +(I(x), I(y)) -> O(+(+(x, y), I(0)))
        *(0, x) -> 0
        *(x, 0) -> 0
        *(O(x), y) -> O(*(x, y))
        *(I(x), y) -> +(O(*(x, y)), y)
        -(x, 0) -> x
        -(0, x) -> 0
        -(O(x), O(y)) -> O(-(x, y))
        -(O(x), I(y)) -> I(-(-(x, y), I(1)))
        -(I(x), O(y)) -> I(-(x, y))
        -(I(x), I(y)) -> O(-(x, y))
        
)