* When multiple elements are equivalent according to {@link Comparable#compareTo}, only the first
   * one specified is included.
   *
   * @throws NullPointerException if any element is null
   * @since 3.0 (source-compatible since 2.0)
   */
  @SuppressWarnings("unchecked")
  public static <E extends Comparable<? super E>> ImmutableSortedSet<E> of(
      E e1, E e2, E e3, E e4, E e5, E e6, E... remaining) {