(defn add-nat [x y]
  (if (zero? x)
    y
    (add-nat (dec x) (inc y))))

(let [orig-add add-nat]
  (binding [add-nat (fn [& args]
                      (apply println "Calling arg-nat with" args)
                      (let [result (apply orig-add args)]
                        (println "Result is" result)
                        result))]
    (add-nat 3 4)))