* NavigableMap methods.
     */

    private final NavigableSet<K> set;
    private final Function<? super K, V> function;

    NavigableAsMapView(NavigableSet<K> ks, Function<? super K, V> vFunction) {
      this.set = checkNotNull(ks);
      this.function = checkNotNull(vFunction);
    }

    @Override
    public NavigableMap<K, V> subMap(
        @ParametricNullness K fromKey,