3 it's still in there and multiply in every appearance of X 3 this one this one and so we have this thing over here now let's look at the quantities we have after we group things this way so let's start again over here what do we get if you look at this quantity over here we looked at it in filtering this is computing P x1 comma Z 0 comma Z one simple recursive calculation then we look at this one over here this quantity here we have not seen it in filtering but we can interpret it what is Z Z for given X for X 4 and X 3 this is really