Clojure Exercise #1: Bend My Mind With MapSet

Write a function, mapset, that works like map except the return value is a set:
(mapset inc [1 1 2 2])
;=> #{2 3}


Answer:

(defn mapset [fn elements]
(let [uniq (into #{} elements)]
(set (map fn uniq))))

How can I write a piece of code without a unit test?

(deftest mapset-test

(is (= #{ 2 3 } (mapset inc [1 1 2 2]))))

Invoking mapset:

(mapset inc [1 1 2 2])

;=> #{3 2}