[tz] Proposed patch - Theory notes for backward file

[Zefram]

Stephen Colebourne wrote:
>My primary concern is ensuring that the backward file is removable -
>ie. no other files contain the "inactive" backward Link entries.

These statements don't match.  For the "backward" file to be removable
what you need is that the "backward" file does not contain any "active"
entries.  No other files containing "inactive" entries is what's needed
to have an easy way of feeding only "active" entries to zic.  Maybe you
want both of these things?

I'd also like to see the active and inactive links distinguished somehow,
for internal clarity.  I'm not so concerned about the mechanism by which
they're distinguished, but segregating them fully by file certainly has
its attractions.  FWIW, my winnowing branch effectively distinguishes
between active and inactive links for geographical zones by whether it
includes population data.  (It doesn't thus distinguish active/inactive
non-geographical zones.)

-zefram