...it is important to note that in the conventional use of reification, the subject of the reification triples is assumed to identify a particular instance of a triple in a particular RDF document, rather than some arbitrary triple having the same subject, predicate, and object. This particular convention is used because reification is intended for expressing properties such as dates of composition and source information, as in the examples given already, and these properties need to be applied to specific instances of triples. There could be several triples that have the same subject, predicate, and object and, although a graph is defined as a set of triples, several instances with the same triple structure might occur in different documents. Thus, to fully support this convention, there needs to be some means of associating the subject of the reification triples with an individual triple in some document. However, RDF provides no way to do this.