code: public <N1 extends N, V1 extends V> ImmutableValueGraph.Builder<N1, V1> immutable() {
      ValueGraphBuilder<N1, V1> castBuilder = cast();
      return new ImmutableValueGraph.Builder<>(castBuilder);
    }
docs: Returns an {@link ImmutableValueGraph#Builder} with the properties of this {@link
  ValueGraphBuilder}.
  
  <p>The returned builder can be used for populating an {@link ImmutableValueGraph}.
  
  @since NEXT