(VAR x y )
(RULES 
        le(0, y) -> true
        le(s(x), 0) -> false
        le(s(x), s(y)) -> le(x, y)
        minus(x, 0) -> x
        minus(0, x) -> 0
        minus(s(x), s(y)) -> minus(x, y)
        gcd(0, y) -> y
        gcd(s(x), 0) -> s(x)
        gcd(s(x), s(y)) -> if_gcd(le(y, x), s(x), s(y))
        if_gcd(true, x, y) -> gcd(minus(x, y), y)
        if_gcd(false, x, y) -> gcd(minus(y, x), x)
        
)