public function addPlaceholder(SourceEvent $event)
{
$source = $event->getTemplate();
$format = $this->placeholderFormat;
$source = $this->processTwig($source, function ($twig) use($format) {
return sprintf($format, htmlspecialchars($twig, ENT_COMPAT, 'UTF-8'));
});
$event->setTemplate($source);
}