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not
x => boolean
x—a generalized boolean (i.e., any object).
boolean—a boolean.
Returns t if x is false; otherwise, returns nil.
(not nil) => T (not '()) => T (not (integerp 'sss)) => T (not (integerp 1)) => NIL (not 3.7) => NIL (not 'apple) => NIL
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.