*/
  public static <T> Comparator<Optional<T>> emptiesFirst(Comparator<? super T> valueComparator) {
    checkNotNull(valueComparator);
    return Comparator.<Optional<T>, @Nullable T>comparing(
        o -> o.orElse(null), Comparator.nullsFirst(valueComparator));
  }

  /**
   * Returns a comparator of {@link Optional} values which treats {@link Optional#empty} as greater