Dynamic tag completion with all tags from all agenda files

[Tassilo Horn <tassilo@member.fsf.org>]

Hi all,

is there a way to get the tag completion suggest all tags from all my
agenda files?

The rationale is that I use `org-remember' very extensively, and I often
want to tag the new entry.  But there's no dynamic completion, because
that uses only the current file's tags and the remember buffer has none.

I could setup all tags in `org-tag-alist', but tags are not really
static.  I could add a new one with TAB in the tag interface, but then I
would have to add it to `org-tag-alist' manually.  Not very nifty...

What would be cool, was that org collected all tags from all agenda
files as soon as the agenda is built the first time.  Then it uses those
cached tags for completion (maybe with some automatic fast selection key
calculation *dreaming*).  When a user adds a new tag with TAB in the tag
interface, it is added to that list.

I don't know exactly how the behavior should be with `org-tag-alist' set
to something.  Maybe a good approach for the whole wish would be adding
a special entry to org-tag-alist:

  ;; Use all tags from all agenda files, and add the tags foo and bar,
  ;; even if they are not used anywhere.
  (setq org-tag-alist '(:all foo bar))

Now only the behavior of the TAGS keyword in files has to be defined
somehow.  Well, I don't have an opinion.  There are two options: 1)
extend the dynamic list of all tags with the ones defined in TAGS
(consistent with the :all keyword above), or 2) the dynamic list is
disabled and only those tags are shown (consistent with the current
behavior).

What do you think?

Bye,
Tassilo