* created for them.
   */
  enum NoRecurse implements Feature<Void> {
    SUBMULTISET,
    DESCENDING;

    @Override
    public Set<Feature<? super Void>> getImpliedFeatures() {
      return emptySet();
    }
  }

  /** Two bounds (from and to) define how to build a subMultiset. */
  enum Bound {
    INCLUSIVE,
    EXCLUSIVE,
    NO_BOUND;
  }

    return forwardMultiset().tailMultiset(toElement, boundType).descendingMultiset();
  }

  @Override
  public SortedMultiset<E> subMultiset(
      E fromElement, BoundType fromBoundType, E toElement, BoundType toBoundType) {
    return forwardMultiset()
        .subMultiset(toElement, toBoundType, fromElement, fromBoundType)
        .descendingMultiset();
  }

  @Override

  }

  @Override
  public SortedMultiset<E> subMultiset(
      @ParametricNullness E fromElement,
      BoundType fromBoundType,
      @ParametricNullness E toElement,
      BoundType toBoundType) {
    return forwardMultiset()
        .subMultiset(toElement, toBoundType, fromElement, fromBoundType)
        .descendingMultiset();
  }

  @Override

  }

  @Override
  public SortedMultiset<E> subMultiset(
      @ParametricNullness E fromElement,
      BoundType fromBoundType,
      @ParametricNullness E toElement,
      BoundType toBoundType) {
    return forwardMultiset()
        .subMultiset(toElement, toBoundType, fromElement, fromBoundType)
        .descendingMultiset();
  }

  @Override

   * works better than Ordering.natural().nullsFirst() because, if null comes before all other
   * values, it lies outside the submap/submultiset ranges we test, and the variety of tests that
   * exercise null handling fail on those subcollections.
   */
  public abstract static class NullsBefore implements Comparator<@Nullable String>, Serializable {

    throw new UnsupportedOperationException();
  }

  @Override
  public abstract ImmutableSortedMultiset<E> headMultiset(E upperBound, BoundType boundType);

  @Override
  public ImmutableSortedMultiset<E> subMultiset(
      E lowerBound, BoundType lowerBoundType, E upperBound, BoundType upperBoundType) {
    checkArgument(
        comparator().compare(lowerBound, upperBound) <= 0,
        "Expected lowerBound <= upperBound but %s > %s",