<<snip much good advice from Hans>> Hi Hans Thank you for your commnts. Oh, for sure, tinkering with plain is just an interim step along the (long...) road. A very minimal "harness setup" to write minimal code --- to play with the various LuaTeX API functions via GNU's gdb debugger. By building a small test environment/playground from scratch, including compiling LuaTeX, and hacking together a basic TDS, gives me a way to learn about stuff like kpathsea etc. Whether or not it's a good way to learn, the right stuff to learn, I'm not sure, but I have learnt a lot through trial + error and not a little frustration, at times... And it keeps me pleasantly occupied with an absorbing and very low-cost hobby :-) If there would be any interest, I'd be happy to write it up at some point if anyone would benefit --- but it'd need scrutiny from resident experts prior to publication: peer review :-) Warm wishes Graham