NEW ISSUE : Extensibility model needs a little fleshing out (processing model, understand/ignore semantics)

Hi Addressers:

As mentioned a few minutes ago at the F2F, I believe we need to
"bookmark" the fact that our extensibility model for EPRs needs a bit of
work, hence this issue.  In particular:

* We discuss the fact that extra elements/attributes might appear in the
infoset of an EPR in section 2.2, but we do not discuss any sort of a
processing model for these extensions.  I would think we would at least
want to describe the fact that extensions in EPRs might cause particular
effects, including for instance adding or modifying abstract properties.

* The abstract properties list might want to be explicitly noted to be
extensible in section 2.1.

* (this relates to processing model) We should decide if we want to
support a "mustUnderstand" marker on extensions a la SOAP/WSDL, or if we
want to discuss "ignoreUnknown" semantics for unrecognized extensions.

Thanks,
--Glen