Read graph and add collections to document

def _read_collections(self, graph):
        """
        Read graph and add collections to document
        """
        for e in self._get_elements(graph, SBOL.Collection):
            identity = e[0]
            c = self._get_rdf_identified(graph, identity)
            members = []
            # Need to handle other non-standard TopLevel objects first
            for m in graph.triples((identity, SBOL.member, None)):
                members.append(self._collection_store[m[2].toPython()])
            obj = Collection(members=members, **c)
            self._collections[identity.toPython()] = obj