code: def get_external_references(self):
          
          for ext_ref_node in self.node.findall('externalReferences'):
              ext_refs_obj = CexternalReferences(ext_ref_node)
              for ref in ext_refs_obj:
                  yield ref
docs: Iterator that returns all the external references of the markable
          @rtype: L{CexternalReference}
          @return: the external references