Issue: ALIST-NIL

[vanroggen%aitg.DEC@decwrl.dec.com]

I've put out an open query of our internal users to see if anyone
actually uses this "feature".  I'd like to support the proposal
ALIST-NIL:DISALLOW.

Another reason for adopting this change is that the xASSOCx functions
would typically be faster in many implementations, since they wouldn't
have to explicitly check for atoms or NIL's.  Since association lists
are often heavily used, the proposal would also have a positive impact
on performance.

			---Walter