protected function shouldUseProxyDigestAuth(HTTP_Request2_Response $response)
    {
        if (407 != $response->getStatus() || !$this->request->getConfig('proxy_user')) {
            return false;
        }
        if (!($challenge = $this->parseDigestChallenge($response->getHeader('proxy-authenticate')))) {
            return false;
        }

        $key = 'proxy://' . $this->request->getConfig('proxy_host') .
               ':' . $this->request->getConfig('proxy_port');

        if (!empty(self::$challenges[$key])
            && (empty($challenge['stale']) || strcasecmp('true', $challenge['stale']))
        ) {
            $ret = false;
        } else {
            $ret = true;
        }
        self::$challenges[$key] = $challenge;
        return $ret;
    }