> For the complete documentation index, see [llms.txt](https://www.oulan.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.oulan.com/2020/03/18.md).

# 18

凡经过的，必有痕迹。愿回首时，彼时或因此时。

![](/files/bNASzmjZ6dQ7iYWbL8Ik)