(defun sum (l) (sum l 0))
(defun sum
  (('() total) total)
  (((cons h t) total) (sum t (+ h total))))

; or using a ``cons`` literal instead of the constructor form:

(defun sum (l) (sum l 0))
(defun sum
  (('() total) total)
  ((`(,h . ,t) total) (sum t (+ h total))))