File Write Permission Rules

[Jon H. LaBadie]

This is a future direction of UNIX question.

I stumbled upon a system with /tmp directory permissions as follows:

	drwsrwxrwx ......

Note the "s". Turns out, the system had a "research facility" added to
the kernel, namely a meaning for the set user id bit on directories.
The meaning?  Kind of neat!  Anyone could create files in the directory,
but only the owner of the directory, OR the owner of the individual
files could remove them.  It was not simply a root facility either,
I could create similar directories and anyone could write to them,
but could not remove anything except their own stuff.

You can imagine the egg on my face when, in an Intro. to UNIX class,
I tried to "prove" that anyone could remove files from a
writable/executable directory with a demo during the lecture.

So, the questions:
	1. Anyone know of the future fate of this useful tweak?
	2. Anyone know of any problems with this idea that suggest
	   it should not be implemented?