[tz] Extended data format

[Andy Lipscomb]

<From: random832 at fastmail.us [mailto:random832 at fastmail.us]
<Sent: Thursday 05 September 2013 13:30
<
<On Thu, Sep 5, 2013, at 13:12, Ian Abbott wrote:
<> Include kalimdor
<>
<> to include the non-extended zones before it defines the extended zones.
<>   Then zic could be run on either kalimdor or kalimdor-extended, but
<> not both, since the latter includes the former automatically.
<
<What about a require/import statement, which parses the given filename but does not generate output?
<
<This would also allow us to centralize Rules that are used in more than one file.

Hmm... this is getting into details of how-the-compiler-works that require more knowledge than I have of the wizardry. As such, I have to write on a slightly more abstract level, but as I see it, there are two key things that have to be avoided, both of which already exist (in simpler form) for links: a reference pointing to a nonexistent zone, or a circular reference.

Also a note with respect to winnowing: if all transitions in a zone are before the cutoff date, and the last line is a straight single reference (=zone = =), then the zone is equivalent to a link.