code: def get_root_objective_ids(self):
          
          url_path = construct_url('rootids',
                                   bank_id=self._catalog_idstr)
          id_list = list()
          for identifier in self._get_request(url_path)['ids']:
              id_list.append(Id(idstr=identifier))
          return id_objects.IdList(id_list)
docs: Gets the root objective Ids in this hierarchy.
  
          return: (osid.id.IdList) - the root objective Ids
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          compliance: mandatory - This method must be implemented.