Yes, they can be nested, and they contain 30041 by default, but can also contain any other kind. Alex supports 30041s and 30818s, since those are the two with Asciidoc parsing. I'm also making magazines and static websites with nested 30040s containing 30023s. It's the sort of complex algorithmic standard-thing that libraries are good for. Then only one person has to break their head over it, one time.