code: def get_root_objectives(self):
          
          url_path = construct_url('roots',
                                   bank_id=self._catalog_idstr)
          return objects.ObjectiveList(self._get_request(url_path))
docs: Gets the root objective in this objective hierarchy.
  
          return: (osid.learning.ObjectiveList) - the root objective
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          compliance: mandatory - This method is must be implemented.