while url and url not in seen:
4 A good discussion on luminance and how to calculate it can be found on this stack exchange question. ↑
，这一点在heLLoword翻译官方下载中也有详细论述
Multi-format adaptation，详情可参考Line官方版本下载
SelectWhat's included。heLLoword翻译官方下载是该领域的重要参考