protected function define_definition_plugin_structure() {

        // Append data only if the grand-parent element has 'method' set to 'rubric'
        $plugin = $this->get_plugin_element(null, '../../method', 'rubric');

        // Create a visible container for our data
        $pluginwrapper = new backup_nested_element($this->get_recommended_name());

        // Connect our visible container to the parent
        $plugin->add_child($pluginwrapper);

        // Define our elements

        $criteria = new backup_nested_element('criteria');

        $criterion = new backup_nested_element('criterion', array('id'), array(
            'sortorder', 'description', 'descriptionformat'));

        $levels = new backup_nested_element('levels');

        $level = new backup_nested_element('level', array('id'), array(
            'score', 'definition', 'definitionformat'));

        // Build elements hierarchy

        $pluginwrapper->add_child($criteria);
        $criteria->add_child($criterion);
        $criterion->add_child($levels);
        $levels->add_child($level);

        // Set sources to populate the data

        $criterion->set_source_table('gradingform_rubric_criteria',
                array('definitionid' => backup::VAR_PARENTID));

        $level->set_source_table('gradingform_rubric_levels',
                array('criterionid' => backup::VAR_PARENTID));

        // no need to annotate ids or files yet (one day when criterion definition supports
        // embedded files, they must be annotated here)

        return $plugin;
    }