code: public function serialize(array $documents)
      {
          $xmlWriter = new XMLWriter();
          $xmlWriter->openMemory();
          $xmlWriter->startElement('add');
  
          foreach ($documents as $document) {
              if (empty($document->documents)) {
                  $document->documents[] = $this->getNestedDummyDocument($document->id);
              }
  
              $this->writeDocument($xmlWriter, $document);
          }
  
          $xmlWriter->endElement();
  
          return $xmlWriter->outputMemory(true);
      }
docs: Create update XML for the given array of $documents.
  
  @param \eZ\Publish\SPI\Search\Document[] $documents
  
  @return string