PHP reference

That's pretty nifty. I'm going to copy that and use the integer division and see how that fares

You don't need to apply map here. Map will pass progressive values accross multiple lists, so long as the function accepts the requisite number of arguments.

(map (fn [x y z] (apply + x y z))
     '(1 2 3)
     '(4 5 6)
     '(7 8 9))

Contrived example, but should illustrate the point.

I love it when digits explode

n means an integer, in all computer science literature and mathematical prose. You're note adding any useful information by spelling it out.

Recursive call not in tail position.