solution accompanying https://groups.google.com/d/msg/clojure/Ksz-dGFnqVE/drHzicm7JxAJ

congeal-consecutives.clj

(ns experimental-clojure.congeal-consecutives)

(def v [1 3 4 5 7 9 10 11 12])

(defn congeal-consecutives [coll]
  (->> coll
       (map-indexed (fn [i x] [(- x i) x]))
       (partition-by first)
       (mapv (fn [pairs]
               (mapv second pairs)))))

(defn rangify [coll]
  (mapv (fn [r]
          (let [f    (first r)
                top  (peek  r)]
            (if (= f top)
              (str f)
              (str f "-" top)))) coll))

(-> v
    congeal-consecutives
    rangify)

Thanks! :)