Defining AVERAGING clause

I was going to define an `AVERAGING` collector clause for iterate, but I'm not sure how to do it. The obvious thing, it seemed to me, would be to sum the values as I go along, and count them, and then divide the sum by the count when leaving the loop. But the examples for `DEFMACRO-CLAUSE` in the manual do all of their work while iterating, and there doesn't seem to be an "at-end" hook. Is the kind of thing I would like feasible, and if so, how is it to be done? thanks! r