(VAR x y )
(RULES 
        f(x, a(b(y))) -> f(a(a(x)), y)
        f(x, b(a(y))) -> f(b(b(x)), y)
        f(a(x), y) -> f(x, a(y))
        f(b(x), y) -> f(x, b(y))
        
)