@abc.abstractmethod<EOL><INDENT>def get_qualifier_hierarchy_session(self, qualifier_hierarchy_id, proxy):<DEDENT>
return<EOL>
Gets the ``OsidSession`` associated with the qualifier hierarchy traversal service.

        The authorization service uses distinct hierarchies that can be
        managed through a Hierarchy OSID.

        :param qualifier_hierarchy_id: the ``Id`` of a qualifier hierarchy
        :type qualifier_hierarchy_id: ``osid.id.Id``
        :param proxy: a proxy
        :type proxy: ``osid.proxy.Proxy``
        :return: a ``QualifierHierarchySession``
        :rtype: ``osid.authorization.QualifierHierarchySession``
        :raise: ``NotFound`` -- ``qualifier_hierarchy_id`` not found
        :raise: ``NullArgument`` -- ``qualifier_hierarchy_id`` or ``proxy`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``Unimplemented`` -- ``supports_qualifier_hierarchy()`` is ``false``

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