NavigableMap<K, V> synchronizedNavigableMap(NavigableMap<K, V> navigableMap) {
    return Synchronized.navigableMap(navigableMap);
  }

  /**
   * {@code AbstractMap} extension that makes it easy to cache customized keySet, values, and
   * entrySet views.
   */
  @GwtCompatible
  abstract static class ViewCachingAbstractMap<