/**
  * Changes the prefix of the selected thread.
  */
 public function changePrefix()
 {
     if (!$this->board->getModeratorPermission('canEditPost')) {
         return;
     }
     $prefixOptions = $this->board->getPrefixOptions();
     if (empty($this->prefix) && !$this->board->prefixRequired || isset($prefixOptions[$this->prefix])) {
         $this->thread->setPrefix($this->prefix);
         if ($this->thread->lastPostTime == $this->board->getLastPostTime()) {
             WCF::getCache()->clearResource('boardData', true);
         }
     }
 }