}

  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E element) {
    return ofInternal(Ordering.natural(), element);
  }

  @SuppressWarnings("unchecked")
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(E e1, E e2) {
    return ofInternal(Ordering.natural(), e1, e2);
  }

  @SuppressWarnings("unchecked")