This is kind of hacky due to our method of first doing the rewrite,
and then picking the slot to rewrite to.  This means that at the time
that we create the rewrite, we don't know the exact location of the
"num_inside" counter, so we have to go back at the end and rewrite it.

We could also switch things to pick the rewrite slot first, but that
is also complicated but perhaps more subtly, since during the course
of the rewrite the chosen slot could have gotten rewritten!

This changeset adds this functionality but doesn't use it for anything.