bug#19468: 25.0.50; UI inconveniences with M-.

[Eli Zaretskii <eliz@gnu.org>]

> From: Vitalie Spinu <spinuvit@gmail.com>
> Date: Wed, 29 Apr 2015 17:00:24 +0200
> Cc: Dmitry Gutov <dgutov@yandex.ru>, 19468@debbugs.gnu.org
> 
> I would like to be able to "link" tag tables.
> 
> Let's say I have two projects A and B. By "linked" tag tables I mean
> that both tag tables are simultaneously accessible from descendant files
> of path/to/A and path/to/B.
> 
> The UI is essentially one command - `etag-link-tag-table`. If I call
> `etag-link-tag-table` from project A and choose a tag table in B then A
> and B tag tables become linked, and vice versa. Simple as that.

The "link" command already exists: just invoke visit-tags-table and
answer YES to the question it asks.  What is missing is the
corresponding "unlink" command, which you didn't mention, but which I
think is required, because at some point you may wish to have Emacs
"forget" about A or B (or both).  (Of course, you can emulate "unlink"
by reloading all the tags tables you need, and answering NO to the
above-mentioned question the first time.  But that's sub-optimal and
requires you to keep notes about which tags tables you have loaded at
all times.)