(VAR x y )
(RULES 
        le(0, y) -> true
        le(s(x), 0) -> false
        le(s(x), s(y)) -> le(x, y)
        pred(s(x)) -> x
        minus(x, 0) -> x
        minus(x, s(y)) -> pred(minus(x, y))
        mod(0, y) -> 0
        mod(s(x), 0) -> 0
        mod(s(x), s(y)) -> if_mod(le(y, x), s(x), s(y))
        if_mod(true, s(x), s(y)) -> mod(minus(x, y), s(y))
        if_mod(false, s(x), s(y)) -> s(x)
        
)