public function generateInvoice(Participant $participant, &$filename)
 {
     $filename = sprintf('factura-%s-%d-%s.pdf', $participant->getRegistration()->getConvention()->getSlug(), $participant->getRegistration()->getId(), $participant->getSlug());
     $html = $this->twig->render('themes/invoice/invoice.html.twig', array('participant' => $participant, 'registration' => $participant->getRegistration()));
     return $this->loggableGenerator->getOutputFromHtml($html);
 }