;; a list-of-number is either
;; empty, or
;; (cons number list-of-number)

;; sum-list:  list-of-number -> number
;; consumes a list of numbers and produces the sum of the numbers in the list
(define (sum-list alon)
  (cond [(empty? alon) 0]
        [(cons? alon) (+ (first alon) (sum-list (rest alon)))]))

(check-expect (sum-list empty) 0)
(check-expect (sum-list (cons 3 empty)) 3)
(check-expect (sum-list (cons 5 (cons 2 (cons 9 empty)))) 16)