今天注意到的一个细节：Mercurial 是不能跟踪空文件夹的。

Mercurial does not track directory information. Instead, it tracks the path to a file. Before creating a file, it first creates any missing directory components of the path. After it deletes a file, it then deletes any empty directories that were in the deleted file's path. This sounds like a trivial distinction, but it has one minor practical consequence: it is not possible to represent a completely empty directory in Mercurial.

于是，用空文件夹来构思项目 TODO 的习惯得改改了。Mercurial 所传递的这个理念很有意思──提醒开发者不要试图开空头支票？