def get_root_subject(self):
        'Returns the BNode which describes the topmost subject of the graph.'

        manifest = URIRef(self.manifest)

        if list(self.rdf.triples((manifest, None, None))):
            return manifest
        else:
            return self.rdf.subjects(None, self.manifest).next()
Returns the BNode which describes the topmost subject of the graph.