* V>} to a {@code NavigableMap<K, V>}. If it permitted this, the returned map's {@code
   * comparator()} method might return a {@code Comparator<? extends K>}, which works only on a
   * particular subtype of {@code K}, but promise that it's a {@code Comparator<? super K>}, which
   * must work on any type of {@code K}.
   *
   * @param map the navigable map for which an unmodifiable view is to be returned