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