*
   * @param comparator the function by which to sort list elements
   * @throws NullPointerException if any element of this iterable is {@code null}
   * @since 14.0 (since 13.0 as {@code toSortedImmutableList()}).
   */
  @SuppressWarnings("nullness") // Unsafe, but we can't do much about it now.
  public final ImmutableList<@NonNull E> toSortedList(Comparator<? super E> comparator) {