code: def _getIdentifier(self, url):
          
          for prefix, namespace in self._rdfGraph.namespaces():
              if namespace in url:
                  return url.replace(namespace, '')
docs: Given a url identifier return identifier portion
          Leverages prefixes already in graph namespace
          Returns None if no match
          Ex.  "http://www.drugbank.ca/drugs/DB01268" -> "DB01268"