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

Marcus Harnisch writes:

 > This is bothering me from a functionality perspective. Someone makes a
 > vague reference to file systems that may exist effectively as
 > subdirectories of a superroot, without giving a concrete example and
 > without stating why that superroot in turn couldn't be part of a
 > supersuperroot and so on. (I don't actually care, just curious)

Doesn't Google like you?  (Just teasing, you have no duty to check.)
On the first page of "filesystem superroot" results there are four
references to superroot that seem to make sense here.  The first is
the obsolete Apollo Domain/OS filesystem, the second is Cygwin (which
presumably means it's embedded in the VFS, but that's just a WAG, I
haven't actually looked it up), the third is Linux NILFS, and the
fourth is the HAMMER2 filesystem associated with Dragonfly BSD.

The latter two are concrete filesystems, and apparently use the
superroot as a place to collect information about internal
filesystems, specifically snapshots in the case of HAMMER2.  So for
these concrete filesystems, the "superroot" concept makes sense, and
it isn't "turtles all the way down".[1]

http://leaf.dragonflybsd.org/mailarchive/users/2012-02/msg00020.html
https://www.kernel.org/doc/Documentation/filesystems/nilfs2.txt

It's not clear to me that Emacsen have any business looking above root
in their ordinary operations, though.  I would implement
expand-file-name consistently with POSIX semantics (since Emacs
*doesn't* implement "superroot" for DOS_NT filesystems!), and provide
a special superroot-directory function (or some such) for those who
really have need.

Regards,

Footnotes: 
[1]  http://evolvingthoughts.net/2011/03/turtles-all-the-way-down/ for
those who haven't seen the phrase before.  And for those who have, and
have 30 minutes to browse, the comments are wonderful!  (Some of them,
anyway. ;-)