There's a second half being written which will do symbolic differentiation, and maybe a few small macros. This is supposed to be a gentle introduction for non-lispers and I seriously think this will be enough for two hours.

But I'm writing something more devious just in case.

Any sufficiently advanced technology is indistinguishable from magic. I do know how it works. I've done my meta-circular interpreters.

I also know there's an abs function. There's a sqrt function too. That's not really the point of the thing.

And I am never going to believe that (+ (* x x) (* y y)) is clearer than xx + yy, even though I do realise that that's only because those rules were drummed into me at school.

More exotic examples of operator precedence, yes, I agree.