refer to: Algorithm to return all combinations of k elements from n
(defn comb [k l]
(if (= 1 k) (map vector l)
(apply concat
(map-indexed
#(map (fn [x] (conj x %2))
(comb (dec k) (drop (inc %1) l)))
l))))
(defn all-subsets [s]
(apply concat
(for [x (range 1 (inc (count s)))]
(map #(into #{} %) (comb x s)))))
; (all-subsets #{1 2 3})
; (#{1} #{2} #{3} #{1 2} #{1 3} #{2 3} #{1 2 3})