This should be a generic feature of XML schemas . So there is a natural correspondence between element type subclassing and support class subclassing, but the tow must remain distinct. Note that without the assurances given by such information you cannot just go around embedding one language in another. A surrounding XML context must have the ability to quote, deny, negate or whatever any element. In fact, nothing in XML says that the menaing of a fragment is not affected by thing anywhere else in a document.
Nothing suggests that the process of removing sub-trees creates a valid document.
An RDF document is grounded when it is a logical experssion in which every bound vaiable if identified by a URI - and every unbound one implies quantification. Every time you look one up in the dictionary all you find are more words. For any set of definitions in one direction, there can always be some reverse definitions which can be seen by others as just as valid. So, while the concept of documents grounded in a given base set is important for interoperability, it must not be seen as a goal to force the semantic web into an acyclic structure. A contacting the server B of the source document of the link and finding the path which B had stored as a way to get from B to the server C of the link's destination document.