}
  }

  /**
   * This method returns a {@code ImmutableSortedMap}, consisting of the entries whose keys are less
   * than {@code toKey}.
   *
   * <p>The {@link SortedMap#headMap} documentation states that a submap of a submap throws an
   * {@link IllegalArgumentException} if passed a {@code toKey} greater than an earlier {@code
   * toKey}. However, this method doesn't throw an exception in that situation, but instead keeps

  /**
   * Returns the single element contained in {@code iterator}.
   *
   * @throws NoSuchElementException if the iterator is empty
   * @throws IllegalArgumentException if the iterator contains multiple elements. The state of the
   *     iterator is unspecified.
   */
  @ParametricNullness
  public static <T extends @Nullable Object> T getOnlyElement(Iterator<T> iterator) {
    T first = iterator.next();
    if (!iterator.hasNext()) {