public function forceAuthentication()
    {
        phpCAS::traceBegin();

        if ( $this->isAuthenticated() ) {
            // the user is authenticated, nothing to be done.
            phpCAS::trace('no need to authenticate');
            $res = true;
        } else {
            // the user is not authenticated, redirect to the CAS server
            if (isset($_SESSION['phpCAS']['auth_checked'])) {
                unset($_SESSION['phpCAS']['auth_checked']);
            }
            $this->redirectToCas(false/* no gateway */);
            // never reached
            $res = false;
        }
        phpCAS::traceEnd($res);
        return $res;
    }