get powerset of set

powerset.clj

(use '[clojure.set]
     '[clojure.test])
(defn powerset [ls]
  (if (empty? ls) '(())
      (union (powerset (next ls))
	    (map #(conj % (first ls)) (powerset (next ls))))))

(deftest test-powerset
  (is (= '(()) (powerset '())))
  (is (= '((1) ()) (powerset '(1))))
  (is (= '((1) (1 2) (2) ()) (powerset '(1 2)))))