* navigable map is accomplished through the returned navigable map (or its views).
   *
   * <p>It is imperative that the user manually synchronize on the returned navigable map when
   * iterating over any of its collection views, or the collections views of any of its {@code
   * descendingMap}, {@code subMap}, {@code headMap} or {@code tailMap} views.
   *
   * <pre>{@code