public function writeXML( \XMLWriter $w, string $elementName = 'ImageSizeReducers' )
   {
      
      if ( ! empty( $elementName ) )
      {
         $w->startElement( $elementName );
      }
      
      foreach ( $this as $reducer )
      {
         if ( \is_null( $reducer ) || ! ( $reducer instanceof ImageSizeReducer ) )
         {
            continue;
         }
         $reducer->writeXML( $w );
      }
      
      if ( ! empty( $elementName ) )
      {
         $w->endElement();
      }
      
   }