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