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 sum-digits (number base)
  (loop for n = number then q
        for (q r) = (multiple-value-list (truncate n base))
        sum r until (zerop q)))