public function flow_verify(Flow $flow)
 {
     if (!$flow->isSessions("_onetimeticket") || $flow->inVars("_onetimeticket") !== $flow->inSessions("_onetimeticket")) {
         $flow->vars("_onetimeticket", $flow->inSessions("_onetimeticket"));
         throw new Exception("invalid ticket");
     }
 }