That's pretty nice. I ultimately only included Y in Heresy as an academic exercise (I see the main purpose of it, if any, as being a playground for learning FP

That's pretty nice. I ultimately only included Y in Heresy as an academic exercise (I see the main purpose of it, if any, as being a playground for learning FP and Lisp concepts); on Racket at least, even with it's very good tail-call optimizations, it wasn't really performant enough by itself to be useful.