[kleiba]

Defining functions and data structures is all that programming is, hence debugging is exactly redefining classes (and functions).

Your question about not needing to restart the runtime ("how often do you have to do this?") seems to imply that you expect the common answer to be "(almost) never" -- that expectation is not surprising since most languages don't even support this kind of debugging!

That's exactly the point of the author: to demonstrate a different way of debugging that's afforded by Lisp and to demonstrate that it can be incredibly powerful. It's just something that is perhaps unfamiliar to programmers who are more used to other programming languages where the debugging facilities are different.