(VAR u v x y z )
(RULES 
        f(f(x, y, z), u, f(x, y, v)) -> f(x, y, f(z, u, v))
        f(x, y, y) -> y
        f(x, y, g(y)) -> x
        f(x, x, y) -> x
        f(g(x), x, y) -> y
        
)