/**
   * Returns an immutable collection of the values in this map, sorted by the ordering of the
   * corresponding keys.
   */
  @Override
  public ImmutableCollection<V> values() {
    return valueList;
  }

  @Override
  ImmutableCollection<V> createValues() {
    throw new AssertionError("should never be called");
  }

  /**

   * @since 12.0
   */
  public static <E extends @Nullable Object> NavigableSet<E> unmodifiableNavigableSet(
      NavigableSet<E> set) {
    if (set instanceof ImmutableCollection || set instanceof UnmodifiableNavigableSet) {
      return set;
    }
    return new UnmodifiableNavigableSet<>(set);
  }

  static final class UnmodifiableNavigableSet<E extends @Nullable Object>