It's not implemented. I have not enough time to add it for now, but I'll add it in a future version. The composer also is missing. I never fill these fields in my ID3 tags, so I didn't feel a need for them.
I think I will add another function to get these additional fields. I want to avoid overloading the current Map.
I don't monitor this forum, so please ask your questions about my libraries in their dedicated thread.