(VAR x )
(RULES 
        f(a, x) -> f(b, f(c, x))
        f(a, f(b, x)) -> f(b, f(a, x))
        f(d, f(c, x)) -> f(d, f(a, x))
        f(a, f(c, x)) -> f(c, f(a, x))
        
)