public function output()
 {
     if (MessageGroups::isDynamic($this->group)) {
         return 'Not supported';
     }
     $ffs = null;
     if ($this->group instanceof FileBasedMessageGroup) {
         $ffs = $this->group->getFFS();
     }
     if (!$ffs instanceof GettextFFS) {
         $group = FileBasedMessageGroup::newFromMessageGroup($this->group);
         $ffs = new GettextFFS($group);
     }
     $ffs->setOfflineMode('true');
     $code = $this->options['language'];
     $id = $this->group->getID();
     $filename = "{$id}_{$code}.po";
     header("Content-Disposition: attachment; filename=\"{$filename}\"");
     return $ffs->writeIntoVariable($this->collection);
 }