protected function define_assignment_subplugin_structure() {

        return false; // This subplugin backup is only one example. Skip it.

        /**
         * Any activity sublugins is always rooted by one backup_subplugin_element()
         * Those elements have some unique characteristics:
         *  - They are, basically, backup_nested_elements
         *  - They cannot have attributes
         *  - They don't have XML representations (only their final/child elements have
         *  - They are able to specify one condition in order to decide if the subplugin
         *    must be processed or no (usually we'll put the "type" condition here, but some
         *    activities, may prefer not to use any condition, see workshop)
         */

        /**
         * Here we are defining the information that will be attached, within the "assignment" element
         * when assignments of type "offline" are sent to backup, so we define the backup_subplugin_element
         * as not having any final element (null) and with the condition of the '/assignment/assignmenttype'
         * being 'offline' (that will be checked on execution)
         *
         * Note that, while, we allow direct "injection" of final_elements at the "assignment" level (without
         * any nesting, we usually pass 'null', and later enclose the real subplugin information into deeper
         * levels (get_recommended_name() and 'config' in the example below). That will make things
         * on restore easier, as far as subplugin information will be clearly separated from module information.
         */
        $subplugin = $this->get_subplugin_element(null, '/assignment/assignmenttype', 'offline');

        /**
         * Here we define the real structure the subplugin is going to generate - see note above. Obviously the
         * example below hasn't sense at all, we are exporting the whole config table that is 100% unrelated
         * with assignments. Take it as just one example. The only important bit is that it's highly recommended to
         * use some exclusive name in the main nested element (something that won't conflict with other subplugins/parts).
         * So we are using 'subplugin_assignment_offline_assignment' as name here (the type of the subplugin, the name of the
         * subplugin and the name of the connection point). get_recommended_name() will help, in any case ;-)
         *
         * All the code below is 100% standard backup structure code, so you define the structure, the sources,
         * annotations... whatever you need
         */
        $assassoff = new backup_nested_element($this->get_recommended_name());
        $config = new backup_nested_element('config', null, array('name', 'value'));

        $subplugin->add_child($assassoff);
        $assassoff->add_child($config);

        $config->set_source_table('config', array('id' => '/assignment/id'));

        return $subplugin; // And we return the root subplugin element
    }