Binx

im trying to create a context free grammar over the language L = {a, b} where b is never trippled Here is what I have so far S-> X | Y X-> bbXY | aX | aY | bb Y-> b | lamda lamda being the empty letter I'm wondering if this is an acceptable solution, I guess im also not sure if b never being trippled means it also cannot be quadrupled which would make the problem easier I suppose.