[tz] data not represented in tzfiles

[Zefram]

Paul Eggert wrote:
>The idea behind the recent changes is that all the rules
>one can write can be expressed in extended-POSIX
>form;

Two periods of DST each year can't be expressed, or two-stage onset
of DST.  Anything involving more than two Rule entries being applicable
up to max_year.

>Does this have anything to do with yearistype.sh?

I hadn't considered year-type rules.  That's another way to confound
POSIX-TZ-ification.

>San Luis's perpetual-DST (before the database changed
>yesterday) we can use TZ='WART4WARST,J1/0,J365/24'.

Emitting that kind of TZ value would be a bad idea.  What you've actually
written there, because of the way the transition time-of-day gets
interpreted, has an hour each year of standard time.  If you fix that
(glossing over the question of whether it can be fixed), you're calling
for two transitions to occur simultaneously, the behaviour of which is
not well defined.

-zefram