def get_group_hierarchy_session(self):
raise Unimplemented()<EOL>
Gets a session for retrieving gropup hierarchies.

        return: (osid.resource.GroupHierarchySession) - ``a
                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``.*