foreign keys to xml_catalog
The current uses of public identifiers in the xml_documents and xml_revisions
tables do not explicity reference xml_catalog with a foreign key. We would
benefit if they did, because then we would be assured that each strongly typed
document in the database was known to us in xml_catalog, which should be a
requirement of being a strongly-typed document. Because the FK would need to
point at xml_catalog.catalog_id, we would need a new field in xml_documents, and
it would have to be synchronized with the xml_documents.docid.