This is a minimal implementation, as DocBook lacks a serious way of encoding all of this. Maybe a <formalpara> could do the trick, but I'd need to find a way to shoehorn a title through the styles (i.e. a first complete tag):
Theorem: Bla bla
<formalpara>
<title>Theorem</title>
<para>Bla bla</para>
</formalpara>
This would also only be a solution for single-paragraph things, as formalpara only allows one paragraph. Or a sidebar, but it's semantically very remote.
That's mostly generating DocBook tags at an inappropriate place with some metadata, rather than outputting whatever you have at your disposal. Far from satisfying, but good enough for a generic tool (see details in the new TODO). Doesn't trigger assertions. Is valid XML.
