we'd say okay this thing here is actually the joint is to join between Z 0 well let's write X first join between X 0 and Z 0 then we multiply with X 1 given X 0 and we sum out over X 0 we've seen this last week this will give us the joint between X 1 and Z 0 then we multiply with Z 1 given X 1 so multiply with this conditional at this point we have C 1 comma X 1 comma Z 0 then we multiply with X 2 given X 1 so here we'd have P X 2 comma Z 1 comma X 1 comma Z 0 then we sum out over X 1 so it disappears after here we have P X 2