BTW there's a 2017 followup called xii-lat.tex: https://github.com/davidcarlisle/dpctex/tree/main/xii-lat (https://mirrors.ctan.org/macros/plain/contrib/xii-lat/xii-la...) — having been done nearly 20 years later, it has even more tricks, so is more "fun" to unpack.
BTW there's a 2017 followup called xii-lat.tex: https://github.com/davidcarlisle/dpctex/tree/main/xii-lat (https://mirrors.ctan.org/macros/plain/contrib/xii-lat/xii-la...) — having been done nearly 20 years later, it has even more tricks, so is more "fun" to unpack.