/**
  * @see EPSInterface::calculate()
  */
 public function calculate($sessionId, $calculation)
 {
     try {
         $calculateStdObject = new stdClass();
         $calculateStdObject->sessionId = $sessionId;
         $calculateStdObject->calculation = $calculation->toStdClass();
         $response = parent::calculate($calculateStdObject);
         if (isset($response->return)) {
             $resultCalculation = new ResultCalculation($response->return);
         } else {
             $resultCalculation = null;
         }
         return $resultCalculation;
     } catch (SoapFault $sf) {
         throw new ServerException($sf);
     }
 }