History lessons

[Tim Smith]

> I have it from a reliable source (Ritchie) that in the original Unix file
> system, the directory structure was an arbitrary graph. It was changed
> to a tree because of the hair involved in consistency checking. As late
> as v6, ln command allowed root to link directories, and across file
> systems. This may have been a Purdue hack, though.

Root can still link directories, as far as the kernel is concerned.  As for
linking across file systems, this must be a Purdue hack, since it is not
possible on ordinary v6,v7,TS 1.?, SIII, and SV for very fundamental reasons.
How did they change the file system to allow this?
-- 
					Tim Smith
				ihnp4!{wlbr!callan,cithep}!tim