}

  private <U extends @Nullable Object> ClosingFuture<U> derive(FluentFuture<U> future) {
    ClosingFuture<U> derived = new ClosingFuture<>(future);
    becomeSubsumedInto(derived.closeables);
    return derived;
  }

  private void becomeSubsumedInto(CloseableList otherCloseables) {
    checkAndUpdateState(OPEN, SUBSUMED);
    otherCloseables.add(closeables, directExecutor());
  }

  /**