'Closed world' assumptions in RDF "Could we not use the XML declaration attribute standalone to determine that a document is self-contained? As in:
<xml version="1.0" encoding="UTF-8" standalone="yes">
That assumption would possibly elimitate the usefulness of that document in a Semantic Web, but could potentially make the 'open world' issue controllable."
RDF/XML is just one serialization of RDF so modifying the XML doesn't change the RDF contained within. While I don't think you can make RDF be closed world, things like ontologies do let you express "all pigs don't fly" or you can perform a query "do any pigs in this graph fly?" which may have been what you wanted anyway.