public function addResponse($response, $url = null)
    {
        if (is_string($response)) {
            $response = self::createResponseFromString($response);
        } elseif (is_resource($response)) {
            $response = self::createResponseFromFile($response);
        } elseif (!$response instanceof HTTP_Request2_Response &&
                  !$response instanceof Exception
        ) {
            throw new HTTP_Request2_Exception('Parameter is not a valid response');
        }
        $this->responses[] = array($response, $url);
    }