Lisp used to specialize in offering extravagantly expensive features, maybe time travel debugging would be a good addition.

> you didn't necessarily change the result of (do-it first)

You're right.

> i think the approach here is to accept that you fixed the bug for the second call, but you will still have to go back and retest the first call.

Gotcha. It looks like a very useful feature. I may actually just try it to try to understand how it works: especially since TFA says the CL integration with Emacs is good (I happen to be an Emacs user).