(VAR f x xs y ys )
(RULES 
        le(0, y) -> true
        le(s(x), 0) -> false
        le(s(x), s(y)) -> le(x, y)
        minus(x, 0) -> x
        minus(s(x), s(y)) -> minus(x, y)
        gcd(0, y) -> 0
        gcd(s(x), 0) -> 0
        gcd(s(x), s(y)) -> if(le(y, x), s(x), s(y))
        if(true, s(x), s(y)) -> gcd(minus(x, y), s(y))
        if(false, s(x), s(y)) -> gcd(minus(y, x), s(x))
        zipWith(f, xs, nil) -> nil
        zipWith(f, nil, ys) -> nil
        zipWith(f, cons(x, xs), cons(y, ys)) -> cons(, zipWith(f, xs, ys))
        gcdlists(xs, ys) -> zipWith(, xs, ys)
        
)