public function append()
    {
        foreach (func_get_args() as $value) {
            if ($value instanceof ResponseInterface
                || $value instanceof \Exception
                || $value instanceof PromiseInterface
                || is_callable($value)
            ) {
                $this->queue[] = $value;
            } else {
                throw new \InvalidArgumentException('Expected a response or '
                    . 'exception. Found ' . \GuzzleHttp\describe_type($value));
            }
        }
    }