public static function list_plan_competencies_returns() {
        $uc = user_competency_exporter::get_read_structure();
        $ucp = user_competency_plan_exporter::get_read_structure();

        $uc->required = VALUE_OPTIONAL;
        $ucp->required = VALUE_OPTIONAL;

        return new external_multiple_structure(
            new external_single_structure(array(
                'competency' => competency_exporter::get_read_structure(),
                'usercompetency' => $uc,
                'usercompetencyplan' => $ucp
            ))
        );
    }