(VAR x y )
(RULES 
        g(0, f(x, x)) -> x
        g(x, s(y)) -> g(f(x, y), 0)
        g(s(x), y) -> g(f(x, y), 0)
        g(f(x, y), 0) -> f(g(x, 0), g(y, 0))
        
)