(defn repeated-until* [element done]
  (reify
    Reader
    (read-bytes [this b]
      (loop [b b, result []]
        (let [[success x b] (read-bytes element b)]
          (if success
            (if (= x done)
              [true result b]
              (recur b (conj result x)))
            [false
             (compose-callback
               this
               (fn [result2 b2]
                 [true (concat result result2) b2]))
             b]))))
    Writer
    (sizeof [_] nil)
    (write-bytes [_ buf vs]
      ;; should perhaps do something smarter if size fixed.
      (loop [chunks [], items vs]
        (if-let [elt (first items)]
          (recur (conj chunks (write-bytes element buf elt))
                 (rest items))
          (concat (conj chunks (write-bytes element buf done))))))))