(VAR x y )
(RULES 
        double(0) -> 0
        double(s(x)) -> s(s(double(x)))
        +(x, 0) -> x
        +(x, s(y)) -> s(+(x, y))
        +(s(x), y) -> s(+(x, y))
        double(x) -> +(x, x)
        
)