code: def get_urn(self):
          
          # TODO: check type
          try:
              type_ctsurn = self.session.get_resource(BASE_URI_TYPES % "CTS_URN"
                                                      , self.session.get_class(surf.ns.ECRM['E55_Type']))
              urn = [CTS_URN(urnstring.rdfs_label.one)
                                  for urnstring in self.ecrm_P1_is_identified_by
                                                              if urnstring.uri == surf.ns.ECRM['E42_Identifier']
                                                              and urnstring.ecrm_P2_has_type.first == type_ctsurn][0]
              return urn
          except Exception as e:
              return None
docs: Assumes that each HucitAuthor has only one CTS URN.