[tz] Rules for TZ+ database

[Paul Eggert]

Stephen Colebourne wrote:
> Its important not to forget the existence of non zic parsers here.

Yes, we need to keep that in mind.

> Whereas if the additional zones were placed in a new file "extended",

Yes, that was in my proposal (since reverted from the experimental repository)
and is also in Zefram's, and I expect we'll do something like that.

> pre-1970 data for
> the existing set of IDs should remain in the main files

That doesn't follow.  If the main files currently have a link,
and we want to turn this into a zone only because of pre-1970
data, we want to keep it a link in the main file, so that we
can support existing implementations that are based on the
typical current practice.