public function convert(Payone_Log4php_LoggerLoggingEvent $event)
 {
     if (isset($this->key)) {
         return $event->getMDC($this->key);
     } else {
         $buff = array();
         $map = $event->getMDCMap();
         foreach ($map as $key => $value) {
             $buff[] = "{$key}={$value}";
         }
         return implode(', ', $buff);
     }
 }