PAC: in the backward case, create an aux. var. for the pac_expectation expression

By the way, refactor the MCE case by merging two routines related to the Z₁ aux. var.
This restores the symmetry with the backward case, now that the latter also has
an aux. var. for the pac_expectation operator. Also store the aux. var. IDs in a
structure common to the backward and MCE cases.