protected function write_numerical_units(array $numericalunits) {

        $this->xmlwriter->begin_tag('numerical_units');
        foreach ($numericalunits as $elementname => $elements) {
            foreach ($elements as $element) {
                $element['id'] = $this->converter->get_nextid();
                $this->write_xml('numerical_unit', $element, array('/numerical_unit/id'));
            }
        }
        $this->xmlwriter->end_tag('numerical_units');
    }