* {@link Runnable}s that close them submitted to each one's closing {@link Executor}.
   */
  @VisibleForTesting
  CountDownLatch whenClosedCountDown() {
    return closeables.whenClosedCountDown();
  }

  /** The state of a {@link CloseableList}. */
  enum State {
    /** The {@link CloseableList} has not been subsumed or closed. */
    OPEN,

    /**