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