Based on http://www.w3.org/2000/07/rdf.xsd derived by Ralph Swick from http://www.oasis-open.org/cover/xmlSchemaForRDF.html originally http://lists.xml.org/archives/xml-dev/200002/msg00618.html by Rick Jelliffe this is a "union type"; use xsi:type in the instance to discriminate between concrete subtypes this is a "union type"; use xsi:type in the instance to discriminate between concrete subtypes. @@hmm... can I mark this as "abstract" somehow?