in this version, we reconstruct and scan the mutable fields of all
regions that occur in an assignment, independently on whether the
region is modified. This avoids a bug in the previous version where
the "left" and "right" subregion lists could have different length.
This also avoids a bug in the version before that, where an upper
region could have a shorter subregion list than one of its subregions.
It is possible to fix those issues in a more efficient manner, but this
seems to make code quite more complex for a non-existent practical gain.