(VAR x y )
(RULES 
        a(0, b(0, x)) -> b(0, a(0, x))
        a(0, x) -> b(0, b(0, x))
        a(0, a(1, a(x, y))) -> a(1, a(0, a(x, y)))
        b(0, a(1, a(x, y))) -> b(1, a(0, a(x, y)))
        a(0, a(x, y)) -> a(1, a(1, a(x, y)))
        
)