Straw-man XML Support for PUBLIC Identifiers and XML Catalogs

I think that your proposal is a good balance of the needs of implementors
and users. Just a few of notes. 

 * as long as we are going to discuss SGML catalogs (and I'm still not sure
if we should), we should discuss how to transmit them for those cases where
authors have provided catalogs as a convenient "default mapping." The SGML
Open spec has a little to say about that.

 * instead of saying: "if an XML processor cannot..." why not say "an XML
processor can use the PUBLIC..., for instance to redirect to a local copy of
the information."

 * the "real" SGML Open spec is quite clear that other resolution mechanisms
(i.e. URN resolvers) may be used before and after the catalog. We must also
make that clear.

 * it isn't clear what the CatKeyword production is for without looking at
SGML Open's docs.

Since the catalog mechanism is totally optional, and overridable, why don't
we move it to an Appendix? I, personally, would be 100% happy with that
solution. The "right" catalog mechanism to use for most applications would
be obvious (avoiding the pre-SGML Open Catalog situation in the SGML world),
but people with legitimate other needs would not be constrained to it.

 Paul Prescod