@abc.abstractmethod<EOL><INDENT>def get_hierarchy_traversal_session_for_hierarchy(self, hierarchy_id, proxy):<DEDENT>
return<EOL>
Gets the ``OsidSession`` associated with the hierarchy traversal service for the given hierarchy.

        :param hierarchy_id: the ``Id`` of the hierarchy
        :type hierarchy_id: ``osid.id.Id``
        :param proxy: a proxy
        :type proxy: ``osid.proxy.Proxy``
        :return: a ``HierarchyTraversalSession``
        :rtype: ``osid.hierarchy.HierarchyTraversalSession``
        :raise: ``NotFound`` -- ``hierarchyid`` not found
        :raise: ``NullArgument`` -- ``hierarchy_id`` or ``proxy`` is ``null``
        :raise: ``OperationFailed`` -- ``unable to complete request``
        :raise: ``Unimplemented`` -- ``supports_hierarchy_traversal()`` or ``supports_visible_federation()`` is ``false``

        *compliance: optional -- This method must be implemented if
        ``supports_hierarchy_traversal()`` and
        ``supports_visible_federation()`` are ``true``.*