code: protected function getMainTranslationDocument(Document $document)
      {
          // Clone to prevent mutation
          $document = clone $document;
          $subDocuments = array();
  
          $document->id .= 'mt';
          $document->fields[] = new Field(
              'meta_indexed_main_translation',
              true,
              new FieldType\BooleanField()
          );
  
          foreach ($document->documents as $subDocument) {
              // Clone to prevent mutation
              $subDocument = clone $subDocument;
  
              $subDocument->id .= 'mt';
              $subDocument->fields[] = new Field(
                  'meta_indexed_main_translation',
                  true,
                  new FieldType\BooleanField()
              );
  
              $subDocuments[] = $subDocument;
          }
  
          $document->documents = $subDocuments;
  
          return $document;
      }
docs: Returns version of the $document to be indexed in the always available core.
  
  @param \eZ\Publish\SPI\Search\Document $document
  
  @return \eZ\Publish\SPI\Search\Document