http://programmingpraxis.com/2012/06/15/counting-ones/

(defun digits (n &optional (b 10))
  (loop for i = n then (floor i b) with lst = ()
     do (push (mod i b) lst)
     while (> i (1- b))
     finally (return lst)))

(defun onep (n) (equal 1 n))
(defun count-ones (n)
  (reduce #'+ (remove-if-not #'onep (digits n))))