*
   * @throws NullPointerException if any of {@code elements} is null
   */
  public static <E> ImmutableSet<E> copyOf(Iterator<? extends E> elements) {
    // We special-case for 0 or 1 elements, but anything further is madness.
    if (!elements.hasNext()) {
      return of();
    }
    E first = elements.next();
    if (!elements.hasNext()) {
      return of(first);
    } else {

   *
   * @throws NullPointerException if any of {@code elements} is null
   */
  public static <E> ImmutableSet<E> copyOf(Iterator<? extends E> elements) {
    // We special-case for 0 or 1 elements, but anything further is madness.
    if (!elements.hasNext()) {
      return of();
    }
    E first = elements.next();
    if (!elements.hasNext()) {
      return of(first);
    } else {