[tz] Extended data format

[Clive D.W. Feather]

Paul Eggert said:
>> I guess such =zone references should refer to a zone earlier in the same file to keep things simple
> 
> But order is not supposed to matter in zic input.
> The man page says "Except for continuation lines,
> lines may appear in any order in the input."
> It's probably better to continue in that tradition,
> even if it's a bit harder to implement.

Right.

We are long past the days when forward references were hard to compute. The
computer can do the work of a two-pass algorithm if it has to. Make it easy
for users to write rather than forcing arbitrary-looking rules.

(There are algorithms for detecting loops, so that's not an argument
either.)

-- 
Clive D.W. Feather          | If you lie to the compiler,
Email: clive at davros.org     | it will get its revenge.
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