private function getContributorXML($contributor)
 {
     $contributor_xml = '';
     if ($contributor->visibility == 1) {
         $dom = new \Podlove\DomDocumentFragment();
         $xml = $dom->createElement('atom:contributor');
         // add the empty name tag
         $name = $dom->createElement('atom:name');
         $xml->appendChild($name);
         // fill name tag with escaped content
         $name_text = $dom->createTextNode($contributor->getName());
         $name->appendChild($name_text);
         if ($contributor->guid) {
             $xml->appendChild($dom->createElement('atom:uri', $contributor->guid));
         }
         $dom->appendChild($xml);
         $contributor_xml .= (string) $dom;
     }
     return $contributor_xml;
 }