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Re: CAR of NIL

Thanks.  I figured there was a semi-sensible-if-archaic explanation for it.
If the thing has to have a CAR as well as a CDR, I guess I'll change my
vote from NIL to ().  From an implementor's standpoint, it's not too tough
for the CDR of NIL to be NIL; just put the value cell of an ID record in
the same position as the CDR cell in a pair record.  It's rather slim
grounds for choosing the layout, but these things tend to be rather
arbitrary anyway.  If it has to have 2 fields dedicated to NIL, things get
hairier.  One could put the property list cell of an ID in the CAR
position, but then of course NIL's real property list has to go somewhere
else, and we need special code in property list accessing for NIL.  If it
has to be special-cased, there's probably a more intelligent way to do it.
I'd rather have a separate data type that looks like a pair, even if it
means losing one more precious tag.