(VAR b x y z )
(RULES 
        le(0, y) -> true
        le(s(x), 0) -> false
        le(s(x), s(y)) -> le(x, y)
        minus(x, x) -> 0
        minus(x, 0) -> x
        minus(0, x) -> 0
        minus(s(x), s(y)) -> minus(x, y)
        isZero(0) -> true
        isZero(s(x)) -> false
        mod(x, y) -> if_mod(isZero(y), le(y, x), x, y, minus(x, y))
        if_mod(true, b, x, y, z) -> divByZeroError
        if_mod(false, false, x, y, z) -> x
        if_mod(false, true, x, y, z) -> mod(z, y)
        
)