@abc.abstractmethod<EOL><INDENT>def get_hierarchy_structure_notification_session(self, hierarchy_structure_receiver):<DEDENT>
return<EOL>
Gets the session for subscribing to notifications of changes within a hierarchy structure.

        :param hierarchy_structure_receiver: a receiver
        :type hierarchy_structure_receiver: ``osid.hierarchy.HierarchyStructureReceiver``
        :return: a ``HierarchyStructureNotificationSession``
        :rtype: ``osid.hierarchy.HierarchyStructureNotificationSession``
        :raise: ``NullArgument`` -- ``hierarchy_structure_receiver`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``Unimplemented`` -- ``supports_hierarchy_structure_notification()`` is ``false``

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