[kragen]

The way I remember it, there was a SET command to set how many versions of a file should be kept, and PURGE would delete all but the most recent version. You could see the date of each version in DIR (they were listed on separate lines) but there wasn't a convenient way to open "FOO.FOR as of yesterday morning". You would have to figure out what version number to ask for and open "FOO.FOR;53" or whatever.

I agree that implementing this functionality in MS-DOS would have been relatively straightforward and acceptably efficient.

It'd be great to have long-lived transactions in a filesystem, permitting higher-level undo and redo.