[tei-council] tricky situations for using @validUntil

[Syd Bauman]

Another possible solution.

4. Rely on <constraintSpec> more
-- ---- -- ---------------- ----
This suggestion is to use <constraintSpec> to generate warnings for
users. We could still use @validUntil to flag stuff for ourselves.
But rather than trying to generate generic messages from a construct
that has @validUntil, we would expect the Council members deprecating
something to write individualized <constraintSpec>s for it. That way
the error messages users get can be tailored. E.g. "use
<listRelation> instead" or "see FR 1234"[1].

To support generation of messages at the right "time" on build, we
would add <constraintSpec> to att.datable. At build time, a
<constraintSpec> that was not "in date" would be ignored.

So:

  <elementSpec ident="relationGrp">
    ...
    <constraintSpec to="2014-12-01" scheme...>
      ... WARN: The 'relationGrp' element will not be available in
      TEI releases after 2014-12-01; TEI recommends the use of
      'listRelation' instead ...
    </>
    <constraintSpec from="2014-12-02" scheme...>
      ... ERROR: The 'relationGrp' element is no longer valid TEI;
      use 'listRelation' instead ...
   </>
  </>