(VAR x y )
(RULES 
        g(x, 0) -> 0
        g(d, s(x)) -> s(s(g(d, x)))
        g(h, s(0)) -> 0
        g(h, s(s(x))) -> s(g(h, x))
        double(x) -> g(d, x)
        half(x) -> g(h, x)
        f(s(x), y) -> f(half(s(x)), double(y))
        f(s(0), y) -> y
        id(x) -> f(x, s(0))
        
)