/**
  * @param SS_HTTPResponse $r
  * @return SS_HTTPResponse|void
  * @throws SS_HTTPResponse_Exception
  */
 public function handleGroupUnselect(SS_HTTPResponse $r)
 {
     if (!Member::currentUser()) {
         return $this->httpError(403);
     }
     $this->presentation->removeFromGroupList();
     return new SS_HTTPResponse("Presentation unselected.", 200);
 }