cljs.user=> (source clojure.set/union)
    (defn union
      "Return a set that is the union of the input sets"
      ([] #{})
      ([s1] s1)
      ([s1 s2]
         (if (< (count s1) (count s2))
           (reduce conj s2 s1)
           (reduce conj s1 s2)))
      ([s1 s2 & sets]
         (let [bubbled-sets (bubble-max-key count (conj sets s2 s1))]
           (reduce into (first bubbled-sets) (rest bubbled-sets)))))
    nil
    cljs.user=> (source clojure.set/intersection)
    (defn intersection
      "Return a set that is the intersection of the input sets"
      ([s1] s1)
      ([s1 s2]
         (if (< (count s2) (count s1))
           (recur s2 s1)
           (reduce (fn [result item]
                       (if (contains? s2 item)
             result
                         (disj result item)))
             s1 s1)))
      ([s1 s2 & sets] 
         (let [bubbled-sets (bubble-max-key #(- (count %)) (conj sets s2 s1))]
           (reduce intersection (first bubbled-sets) (rest bubbled-sets)))))
    nil