[tz] Proposed patch - Theory notes for backward file

[Paul Eggert]

Stephen Colebourne wrote:
> Right now we have three things:
> - Zone entries
> - active Link entries, for locations that happen to have the entire
> local time history of another zone
> - inactive Link entries, that used to have meaning but are no longer
> favoured (the backward file)

It's not that simple.  We also have aliases in the non-backward files.
Two examples are 'Europe/Nicosia' and 'GMT'.  I don't see why these
names should be considered canonical; although they are active link
entries, they are not for differing locations.