This call to simplify_assuming is entirely unneeded, never does the correct thing (unless it fails to do anything), and when it succeeds in doing something, it does the wrong thing (it is being called on the conditions of a piecewise).
This call to simplify_assuming is entirely unneeded, never does the correct
thing (unless it fails to do anything), and when it succeeds in doing something,
it does the wrong thing (it is being called on the conditions of a piecewise).

In particular, this causes `size(a)=size(b) And Q` to become `Q` and breaks
simplification of gmm_gibbs.

zaxtax/hakaru,zachsully/hakaru,zaxtax/hakaru,zachsully/hakaru