A number of goals are converging on the general requirement that we need
a modular Pandoc (https://en.wikipedia.org/wiki/Pandoc) clone written in
portable Scheme. I've tried to avoid it, but it's time to swallow down
the medicine.

I'll work on it piecemeal here and there; help is welcome. It should
probably work by using SXML (http://okmij.org/ftp/Scheme/SXML.html) as
the internal representation, and convert Markdown/Texinfo/etc. to SXML
and back.