next we give an X to graze X 3 to the front P X 3 Z 3 Z 4 given X 2 then we sum out X 3 and we're left with P z3 z4 given x2 so from one side of the chain we get the probability of the evidence that comes after x2 given x2 that's living here over here we have to join between X 1 Z 0 Z 1 but actually we can multiply this thing and some out over X 1 so we'll get P x2 comma Z 0 z 1 so you have the joint of x2 with the past evidence then we have bringing in the current evidence at time 2m we know the conditional submission for the later