Re: (expand-file-name "/..")

I don't think the “superroot” in the comment refers to Windows, else it would have been mentioned somehow. As a matter of fact, looking over the fence revealed a bit more information[1]: /* `/../' is the "superroot" on certain file systems. Turned off on DOS_NT systems because they have no "superroot" and because this causes us to produce file names like "d:/../foo" which fail file-related functions of the underlying OS. (To reproduce, try a long series of "../../" in default_directory, longer than the number of levels from the root.) */