public function addIdref($id, $liner = 'yes', array $properties = [] ){
		$itemref = $this->dom->spine->addChild('itemref');
		$itemref['idref'] = $id;
		if( 'no' === $liner ){
			$itemref['linear'] = 'no';
		}
		if( !empty($properties) ){
			$itemref['properties'] = implode(' ', $properties);
		}
		return $itemref;
	}