(defn mcons [first next]
  {:first (ref first)
   :next (ref next)})

(def L1 (mcons 1 (mcons 2 (mcons 3 nil))))
(def L2 (mcons 4 nil))

(defn mlast-cell [mcol]
  (cond (empty? mcol) nil
        (empty? (deref (:next mcol))) mcol
        :else (mlast-cell (deref (:next mcol)))))

(defn append! [l1 & lists]
  (if (empty? lists)
    l1
    (let [head (first lists)
          r (rest lists)
          l (mlast-cell l1)]
      (if (nil? l1)
        (apply append! lists)
        (do
          (dosync
           (ref-set (:next l) head))
          (apply append! lists))))))