http://lujun9972.github.io/blog/2016/11/18/emacs%E4%B8%AD%E9%82%A3%E4%BA%9B%E4%B8%8D%E5%B8%B8%E7%94%A8%E7%9A%84%E8%A1%8C%E6%93%8D%E4%BD%9C%E5%91%BD%E4%BB%A4/
https://chriszheng.science/2016/09/04/Emacs-remove-empty-lines/
