(not nil)
→(not '())
→(not (integerp 'sss))
→(not (integerp 1))
→(not 3.7)
→(not 'apple)
→
None.
None.
None.
not is intended to be used to invert the `truth value' of a boolean (or generalized boolean) whereas null is intended to be used to test for the empty list. Operationally, not and null compute the same result; which to use is a matter of style.
\issue{NOT-AND-NULL-RETURN-VALUE:X3J13-MAR-93}