If you've never done Common Lisp before, it's better to start by looking at `loop`, which is a built-in macro that provides a bunch of different ways to loop/it

If you've never done Common Lisp before, it's better to start by looking at `loop`, which is a built-in macro that provides a bunch of different ways to loop/iterate: http://www.gigamonkeys.com/book/loop-for-black-belts.html http://www.gigamonkeys.com/book/loop-for-black-belts.html iterate is a Common Lisp library that aims to be a replacement for loop, being cleaner (subjectively) and portably extensible: https://common-lisp.net/project/iterate/ https://common-lisp.net/project/iterate/ `repeat` is an iterate clause that tells iterate to perform N iterations of the loop: (iterate (repeat 3) (print 'beep)) BEEP BEEP BEEP As for this particular `for`, it's an iterate clause that handles numeric iteration (among other things). So: (iterate (for x :from 0) (for y :from 10 :downto 7) (print (list x y))) Would give you: (0 10) (1 9) (2 8) (3 7) iterate will terminate the loop the first time any one of its clauses runs out of values/iterations. So in the post I had something like (iterate (repeat N) (for x :from start) ...) You could write this as (iterate (for x :from start :below (+ start N)) ...) But manually calculating the ending value seems ugly to me. I know I want the loop to happen N times so I just say `(repeat N)` and let the computer figure out when it's time to be done.