code: public <N1 extends N, E1 extends E> ImmutableNetwork.Builder<N1, E1> immutable() {
      NetworkBuilder<N1, E1> castBuilder = cast();
      return new ImmutableNetwork.Builder<>(castBuilder);
    }
docs: Returns an {@link ImmutableNetwork#Builder} with the properties of this {@link NetworkBuilder}.
  
  <p>The returned builder can be used for populating an {@link ImmutableNetwork}.
  
  @since NEXT