return headSet(toElement, false);
  }

  /**
   * @since 12.0
   */
  @Override
  public ImmutableSortedSet<E> headSet(E toElement, boolean inclusive) {
    return headSetImpl(checkNotNull(toElement), inclusive);
  }

  /**
   * {@inheritDoc}
   *
   * <p>This method returns a serializable {@code ImmutableSortedSet}.
   *

        }
      };
    }

    ImmutableSortedSet<C> subSet(Range<C> range) {
      return subRangeSet(range).asSet(domain);
    }

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

    @Override
    ImmutableSortedSet<C> subSetImpl(