@Override
  public ContiguousSet<C> subSet(C fromElement, C toElement) {
    checkNotNull(fromElement);
    checkNotNull(toElement);
    checkArgument(comparator().compare(fromElement, toElement) <= 0);
    return subSetImpl(fromElement, true, toElement, false);
  }

  /**
   * @since 12.0
   */
  @GwtIncompatible // NavigableSet
  @Override
  public ContiguousSet<C> subSet(

  @Override
  public ContiguousSet<C> subSet(C fromElement, C toElement) {
    checkNotNull(fromElement);
    checkNotNull(toElement);
    checkArgument(comparator().compare(fromElement, toElement) <= 0);
    return subSetImpl(fromElement, true, toElement, false);
  }

  /**
   * @since 12.0
   */
  @GwtIncompatible // NavigableSet
  @Override
  public ContiguousSet<C> subSet(

    @Override
    ImmutableSortedSet<C> headSetImpl(C toElement, boolean inclusive) {
      return subSet(Range.upTo(toElement, BoundType.forBoolean(inclusive)));
    }

    @Override
    ImmutableSortedSet<C> subSetImpl(
        C fromElement, boolean fromInclusive, C toElement, boolean toInclusive) {
      if (!fromInclusive && !toInclusive && Range.compareOrThrow(fromElement, toElement) == 0) {
        return ImmutableSortedSet.of();