[tei-council] tricky situations for using @validUntil

[Syd Bauman]

> My sense was that because the @validUntil date is actually passed
> by the time the schema is generated, the generation would actually
> fail -- in other words, not a warning but an error. I had assumed
> that the processing of the @validUntil date was a) based on the
> date of processing, i.e. the date on which you're trying to
> generate the schema, and b) produces an outright error at any point
> beyond that date (as #3 above suggests). Am I wrong about that?

That's exactly what happens when building P5, but not what happens
when generating a customization.

I just ran `roma` on a couple of tiny customizations that included
validUntil=2012-12-12 on a couple of <elementSpec mode=add>s. The
result was that the Schematron was correctly generated and stuck in
the the .isosch file.

However, my test brought out two other issues:
* The Schematron did not make it into the .rng file.
* When the customization-added element was in its own namespace (as
  it should be), the Schematron wouldn't work, as it presumes the TEI
  namespace. This is a known problem. Here's a comment from
  odds/extract-isosch.xsl: 
     This is a start at how we might process other namespaces; doesn't
     work yet for a variety of reasons, but also is not important yet,
     as @validUntil is used by TEI-C only on its own constructs