Missing functionality in forvec()

[Ilya Zakharevich <nospam-abuse@ilyaz.org>]

I want to use forwec() in a complicated algorithms needing a lot of
cached data.  The caches should be recalculated on backtracking.

  Here I consider forvec() as if it is traversing-in-order an “ordered rooted tree”.

While forvec() takes a lot of workload under the cover, with forvec()
it is a PITA to detect backtracking.  On the other hand, I can see
that with an extra argument taking an (optional) variable reference
'v, and reporting the details of backtracking in v, the life would be
much easier!

The docs could go like this (I would need only one of these 3 flavors
of reporting; two others are added in anticipation of usability in
other situations):

  Let n be the number of tail counters which wrapped around after the
  preceding iteration, and m be the number which will wrap around on
  the next iteration.  If 'v was a small/v/vector at start, put n and
  m into the first two components; otherwise report n or -m,
  preferring the non-0 value.  (For what follows, note that with
  flag=2 both n and m may be non-0 even if the trailing ranges are not
  of length 1.)  If 'v was 0 at start, prefer the largest of n and m,
  otherwise prefer positive or negative depending on the sign of 'v at
  start.

Can it be useful in other situations than what I have in mind?  (This
improving the chance of it being implemented. ;―])

Thanks,
Ilya