@abc.abstractmethod<EOL><INDENT>def get_gradebook_hierarchy_session(self):<DEDENT>
return<EOL>
Gets the session traversing gradebook hierarchies.

        :return: a ``GradebookHierarchySession``
        :rtype: ``osid.grading.GradebookHierarchySession``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``Unimplemented`` -- ``supports_gradebook_hierarchy() is false``

        *compliance: optional -- This method must be implemented if
        ``supports_gradebook_hierarchy()`` is true.*