join-all.clj

(defn join-all
  "Join the lists provided using f to select an element each time"
  [f & lists]
  (let [l (remove empty? lists)]
    (when-not (empty? l)
      (let [n (reduce f (map first l))
	    c (count (filter (partial = true) (map (fn [x] (= (first x) n)) l)))
	    r (map (fn [x] (if (= (first x) n) (rest x) x)) l)]
	(lazy-seq
	  (concat
	   (repeat c n)
	   (apply join-all (cons f r))))))))