Example: computing the totals of items in a list

;; compute the total of the mpg consumption for all cars int the loa list
;; List-of-Auto -> Number
;; (define (total-mpg loa)...

;; Examples:
;; (= (total-mpg mt-loa) 0)
;; (= (total-mpg loa1) 20)
;; (= (total-mpg loa2) 30)
;; (= (total-mpg loa3) 45)

;; Template:
;; (cond
;;   [(empty? loa) ...]
;;   [else ... (first loa) 
;;             ... (auto-model (first loa)) ...
;;             ... (auto-tank-size (first loa))...
;;             ... (auto-mpg (first loa)) ...
;;         ... (total-mpg (rest loa)) ...]

;; Program:
(define (total-mpg loa)
  (cond
   [(empty? loa) 0]
   [else (+ (auto-mpg (first loa)) (total-mpg (rest loa)))]))

;; Tests:
(= (total-mpg mt-loa) 0)
(= (total-mpg loa1) 20)
(= (total-mpg loa2) 30)
(= (total-mpg loa3) 45)