Let me add several notes (but no spoilers): One part of the exercise is designing an approach to process sequences generically that can work with the Loop Facility. Dave Tenny's post upthread describes one possibility. Another part is in working out how to extend the Loop Facility itself so one can say (loop for x some-sequence ...) where coming up with a suitable keyword for the placeholder is a subpart of the exercise. Yet another part is thinking about the trade-offs involved in introducing such a subclause. This includes understanding how hardware and compiler technologies have changed in about half a century since the Loop Facility was first designed, so reasonable trade-offs have also changed. And a metanote repeating the well-known: the value of such exercises is in _doing_ them, not in obtaining the final result. #CommonLisp #ComputerProgramming #Lisp #ProgrammingLanguageDesign @davetenny@hachyderm.io