[ebxml-cppa] Placeholder values in CPA templates

[Dale Moberg <dmoberg@cyclonecommerce.com>]

While working on updates to the CPA formation appendix, I realized
that I did not recall that we have every made a decision about how
to mark placeholder values in a CPA template.

Some placeholder values might have a specific type according to schemas,
and in general I doubt that it will work to just omit values (have empty
strings or whatever datatype is involved.)

If no one can find where we have said something about this, I propose
that we add an issue to the database that says how to mark placeholder
values. 

Some more specific questions arise for specific places that might be
subject to placeholders. For example, in PartyId, can we allow the
"type"
attribute to have a placeholder value? Or should a CPA template always
nail down the expected type (eg, "urn:oasis:names:tc:ebxml-cppa:duns")?

Thoughts?