return Comparator.<Optional<T>, @Nullable T>comparing(
        o -> orElseNull(o), Comparator.nullsLast(valueComparator));
  }

  @IgnoreJRERequirement // helper for emptiesFirst+emptiesLast
  /*
   * If we make these calls inline inside the lambda inside emptiesFirst()/emptiesLast(), we get an
   * Animal Sniffer error, despite the @IgnoreJRERequirement annotation there. For details, see
   * ImmutableSortedMultiset.
   */