Modern Common Lisp with FSet

Looks good.

Re https://fset.common-lisp.dev/Modern-CL/Top_html/Sets-Tutorial.html

I find the arb operation on sets a bit weird: wouldn't it be better if there was a function that returned an arbitrary element and a new set without that element?

That way, you wouldn't have to call less afterwards, which is probably inefficient, since it needs to find the element to remove again.