Checking for NIL

The predicates null and not act identically. Good programming style dictates that you use null when the semantics of the program suggest interest in whether a list is empty, otherwise use not:

> (null nil)
T

> (not nil)
T

> (null ())
T

> (not ())               ;;preferable to use null
T

> (null '(a s))
NIL

> (not '(a s))           ;;preferable to use null
NIL

> (not (= 1 (* 1 1)))
NIL

> (null (= 1 (* 1 1)))   ;;preferable to use not
NIL