return new NavigableAsMapView<>(set, function);
  }

  private static class AsMapView<K extends @Nullable Object, V extends @Nullable Object>
      extends ViewCachingAbstractMap<K, V> {

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

    Set<K> backingSet() {
      return set;
    }

    AsMapView(Set<K> set, Function<? super K, V> function) {
      this.set = checkNotNull(set);