* value.
   *
   * @since 22.0 (but only since 33.4.0 in the Android flavor)
   */
  public static <T> Comparator<Optional<T>> emptiesFirst(Comparator<? super T> valueComparator) {
    checkNotNull(valueComparator);
    return Comparator.<Optional<T>, @Nullable T>comparing(
        o -> orElseNull(o), Comparator.nullsFirst(valueComparator));
  }