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