The comma 'unquotes' the place variable so it is replaced with the value passed to the first argument on set-sqrt. I'm not in front of a lisp interpreter, but I think both those v's should be unquoted too in order to get the expected behavior ... (plus they are a bit leaky since they are evaluated twice, but that's just being pedantic).

Read http://www.gigamonkeys.com/book/macros-defining-your-own.htm... for a more thorough explanation.

As for setf, a user can create arbitrary 'setters' using syntax like that described at http://gigamonkeys.com/book/object-reorientation-classes.htm...