I do prefer nil being the false value as well as the empty list, even if it makes it more awkward to distinguish between 'there is a result, but the result is an empty list' and 'there are no results'. But that has nothing to do with car and cdr in Common Lisp treating nil as though it were `(cons nil nil)'. The only value in that I can see is would be if rplaca and rplacd can do some useful things with that (so `(setf (car symbol-that-currently-points-at-nil) foo)' and `(setf (cdr stcpat) bar)' do those useful things).