docs: def get_repository_hierarchy_design_session(self):
          """Gets the repository hierarchy design session.
  
          return: (osid.repository.RepositoryHierarchyDesignSession) - a
                  ``RepostoryHierarchyDesignSession``
          raise:  OperationFailed - unable to complete request
          raise:  Unimplemented -
                  ``supports_repository_hierarchy_design()`` is ``false``
          *compliance: optional -- This method must be implemented if
          ``supports_repository_hierarchy_design()`` is ``true``.*
  
          """
          if not self.supports_repository_hierarchy_design():
              raise errors.Unimplemented()
          # pylint: disable=no-member
          return sessions.RepositoryHierarchyDesignSession(runtime=self._runtime)