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