I’ve written a DTD for an XML Nonpareil state save file format, and code to read and write such files using libxml2. It was very easy. It’s not yet integrated into the main Nonpareil program, because it will need the new memory API and pseudo-reflection. In the mean time, I wrote a utility that can convert the XML state save file format to and from the old NSIM simulator’s state save file format.
XML is more verbose than the NSIM format, but because the XML files are automatically compressed on the fly, the new state files end up being 25 to 50% of the size they use in NSIM format.