[no subject]

[GLS at MIT-MC (Guy L. Steele, Jr.)]

More generally, the suggestion should be that ATOM be used in
preference to NULL in most functions that traverse lists.  This should
probably be true in most NIL implementations, as a mater of error-checking
if nothing else.  On the PDP-10 NULL tests are cheaper than ATOM tests.
Even in NIL there is the question of whether we want to *define*
LENGTH, LAST, etc. to use ATOM tests (which would permit the loser to depend
on it), against the possibility that on some NIL implementation
a NULL test might be significantly faster than an ATOM check.
I would lean toward indeed so defining them to use ATOM, but
want to raise that issue.