public ImmutableSortedSet<E> elementSet() {
    return elementSet;
  }

  @Override
  public ImmutableSortedMultiset<E> headMultiset(E upperBound, BoundType boundType) {
    return getSubMultiset(0, elementSet.headIndex(upperBound, checkNotNull(boundType) == CLOSED));
  }

  @Override
  public ImmutableSortedMultiset<E> tailMultiset(E lowerBound, BoundType boundType) {
    return getSubMultiset(