Task

Take a   Natural Number   in a given base and return the sum of its digits:

   110         sums to   1
   123410   sums to   10
   fe16       sums to   29
   f0e16     sums to   29


(defun digit-sum (n)
  (apply #'+ (mapcar (lambda (c) (- c ?0)) (string-to-list "123"))))
 
(digit-sum 1234) ;=> 10