; prefixs-sums : list-of-num -> list-of-num
; Returns the list of all partial sums of the numbers.
(define (prefix-sums l)
  (cond [(empty? l) empty]
	[(cons? l)  (prefix-sums-help (rest l) (first l))]))

; prefixs-sums : list-of-num num -> list-of-num
; Returns the list with
; * the number
; * all the partial sums of the numbers in the list, except with the number
;   added to each partial sum
(define (prefix-sums-help l sum-so-far)
  (cond [(empty? l) empty]
	[(cons? l)  (cons sum-so-far
			  (prefix-sums-help (rest l)
					    (+ sum-so-far (first l))))]))


(equal? (prefix-sums (list 1 8 7 3)) (list 1 9 16 19))
(equal? (prefix-sums (list 1 1 1 1)) (list 1 2 3 4))