(use '[clojure.test])
(defn powerset
  ([ls] (lazy-seq (cons () (powerset '(()) ls))))
  ([acc ls]
     (if (empty? ls)
       ()
       (let [fs (first ls)
	     added (map #(conj % fs) acc)]
	 (lazy-cat added
		 (powerset (concat acc added) (rest ls)))))))

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