defn combinations items let size count items if size set items if map

(defn combinations
	[items r]
		(let [size (count items)]
			(if (= r size)
				[ (set items) ]
				(if (= 1 r)
					(map #(set [%]) items)
					(let [fst #{ (first items) } rst (rest items)]
						(concat
							(map #(union fst %) (combinations rst (dec r)))
							(combinations rst r)))))))