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I spotted what I think is a bug in the Chapter 5 notebook. In the book, sections 5.19-5.22 do counterfactual modeling with a model where A -> M and A -> D <- M. The predictor A is tweaked and the total effect on D is observed. This total effect comes from the direct link between A and D, but is also mediated by an indirect link via M. That is, values of M need to be simulated as well and their effect on D.
The bug was that it appeared as though the values for M were not being simulated. The difference in the resulting plots in 5.22 is small, but from a modeling perspective it's quite distinct.
This PR isn't quite ready for merge, because my changes break subsequent code from 5.23 onwards. But I am putting the PR up now in the hope of getting feedback on whether I'm doing it the correct way before changing the subsequent code.
Note I haven't run pre-commit checks yet, so don't merge in current state.