[tz] A simple solution to have zones merge into another zone

[Paul Eggert]

Adding a "follow"-like syntax is a fine idea. I think it's been proposed 
before, but not implemented. I would hope that one could use "follow" 
not only at the end of a Zone, but also within a zone, such as "location 
X follows location Y from 1950 to 1990". Also, the code should defend 
itself against follow cycles (e.g., A follows B follows C follows A).

Any changes along these lines would be complicated by 
backward-compatibility concerns, though. That is, we could use a 
"follow"-like syntax only in vanguard format at first. To give you a 
feel of how long this can take, we added support for %z in zone 
abbreviations in 2015f, and we're not using this feature even now in the 
vanguard data, much less the main data (this is mostly due to my lack of 
time...).