This is cool, but it would be even cooler if it could store a list of all changes since last compile. That way, I could say "M-x slime-show-diffs" or some equivalent, and get something like a grep buffer of all changes, with the overlays turned on.
Anybody else think so? I'm going to start something like this, but I'm at work right now, and my elisp is weak, so I don't know how long it will take.