protected static function define_other_properties() {
        return [
            'currentversion' => [
                'type' => policy_version_exporter::read_properties_definition(),
                'null' => NULL_ALLOWED,
            ],
            'draftversions' => [
                'type' => policy_version_exporter::read_properties_definition(),
                'multiple' => true,
            ],
            'archivedversions' => [
                'type' => policy_version_exporter::read_properties_definition(),
                'multiple' => true,
            ],
        ];
    }