First off, thank you for finally getting me to read that, it’s been sitting around in my to-read list since forever (2017? but the version is 2.0 from 2010? a long time ago, anyway).
Second... OK, I got through the first part (the language description), and it basically confirms my earlier brief glances at various Algol 68 documents: it’s a fundamentally pretty nice and consistent language with some warts, some hilariously archaic points, and godawful one-of-a-kind terminology, and its age excuses basically all of those. (No surprise I liked it though, I like both Standard ML and METAFONT, and those are both transparently Algolish in different ways.) Not looking forward to the ALLCAPS operators, but then I wrote some all-caps Forth some time ago and it was okay.
Third, though—that description (unlike the report) doesn’t seem to be using W-grammars at all? It uses what is pretty pedestrian ABNF in funny notation. It’s very helpful in getting me to understand how Algol 68 works, but doesn’t really relate to my original question of how those two-level W-things work. Except by maybe reverse engineering them by comparing the report with the guide, but that sounds like a last-resort option.
[1] https://jmvdveer.home.xs4all.nl/en.download.learning-algol-6...