(if-not test then) (if-not test then else)
Evaluates test. If logical false, evaluates and returns then expr, otherwise else expr, if supplied, else nil.
user=> (defn has-neg [coll]
(if-not (empty? coll) ;; = (if (not (empty? coll)) ...
(or (neg? (first coll)) (recur (rest coll)))))
#'user/has-neg
user=> (has-neg [])
nil
user=> (has-neg [1 2 -3 4])
true
user=> (if-not (zero? 0) :then :else)
:else
;; See examples for "if" explaining Clojure's idea of logical true
;; and logical false.