RDF does not contain id or idRef attributes.

If a RDF-NML description needs to be translated to XML-NML, it is unclear when "id" should be used and when "idRef"?

So either we:

1. Add some additional info to RDF which determines when id and when idRef should be used in XML.
2. pick id or idRef in XML purely based on the syntax, but not on any semantics (e.g. an XML element without children 
uses idRef, an XML element with children uses id)
3. Remove "idRef" and just use "id" everywhere