Would you mind expanding a bit more one one of the ideas you find novel and interesting, for example the stack as the source of truth for all computation? I ha

Would you mind expanding a bit more one one of the ideas you find novel and interesting, for example the stack as the source of truth for all computation? I had a look at the examples section on the bel post and read a few pages into the bel document, but all I saw is basically scheme with a slightly funky syntax and primitives (like where). So I guess pg must really be burying the lede (which, from what I gather from the above has at least partly to do with a elegant way to model concurrency, which ur-lisp lacked?).