17.2.2.1 Examples of Satisfying a One-Argument Test

 (count-if #'zerop '(1 #C(0.0 0.0) 0 0.0d0 0.0s0 3)) =>  4

 (remove-if-not #'symbolp '(0 1 2 3 4 5 6 7 8 9 A B C D E F))
=>  (A B C D E F)
 (remove-if (complement #'symbolp) '(0 1 2 3 4 5 6 7 8 9 A B C D E F))
=>  (A B C D E F)

 (count-if #'zerop '("foo" "" "bar" "" "" "baz" "quux") :key #'length)
=>  3