for here if you're asked if in this factorization order 2 4 3 1 if you're asked to predict X 3 you're allowed to look at X 2 and X 4 if you're asked to predict X 1 you're allowed to look at X 2 X 4 and X 3 so if you only have a single pass through the model the question is do you now input X 3 or do you not because the prediction of X 3 is not allowed to look at X 3 while the prediction of X 1 is allowed to look at X 3 so they do an architectural change in order to achieve both things so that you can do have a