@abc.abstractmethod<EOL><INDENT>def get_group_hierarchy_session(self):<DEDENT>
return<EOL>
Gets a session for retrieving gropup hierarchies.

        :return: ``a GroupHierarchySession``
        :rtype: ``osid.resource.GroupHierarchySession``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``Unimplemented`` -- ``supports_group_hierarchy()`` is ``false``

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