@Override
    public SortedMap<K, V> tailMap(K fromKey) {
      return tailMap(fromKey, true);
    }

    private static final long serialVersionUID = 0;
  }

  @GwtIncompatible // works but is needed only for NavigableMap
  @CheckForNull
  private static <K extends @Nullable Object, V extends @Nullable Object>
      Map.Entry<K, V> nullableSynchronizedEntry(