[tz] Proposed patch - Theory notes for backward file

[Stephen Colebourne]

On 5 September 2013 13:15, Paul Eggert <eggert at cs.ucla.edu> wrote:
> 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.

My primary concern is ensuring that the backward file is removable -
ie. no other files contain the "inactive" backward Link entries. Its
this that I want to see documented more than anything else.

Beyond that, I think consumers will (and already do) treat the
backward file as a source of canonicalization, simply as a result of
the above. I think that the ability to use it as such should be
documented, but I can live with the minimal change proposed in the
paragraph above, for example:

"The file 'backward' consists of Link entries mapping two names. These
are typically interpretted as a link to a modern name from an older
name. If the data set is used with the 'backward' file file excluded,
then it must remain logical and complete."

Stephen