* {@code k} to value {@code v} in the specified map. The behavior of this operation is undefined
   * if the specified map is modified while the operation is in progress.
   */
  public void putAll(Map<? extends K, ? extends Long> m) {
    for (Entry<? extends K, ? extends Long> entry : m.entrySet()) {
      put(entry.getKey(), entry.getValue());
    }
  }

  /**