;; A ListOfNumber is either
;; -- empty
;; (cons Number ListOfNumber)

#|
(define (f lon)
  (cond
    [(empty? lon) ...]
    [(cons? lon) ... (first lon) ... (f (rest lon)) ...]))
|#

;; negate: ListOfNumber -> ListOfNumber
;; negates all numbers
(define (negate lon)
  (cond
    [(empty? lon) empty]
    [(cons? lon) (cons (* -1 (first lon))
                           (negate (rest lon)))]))

;; Tests
(equal? (negate empty) empty)
(equal? (negate (cons 1 empty)) (cons -1 empty))
(equal? (negate (cons -1 empty)) (cons 1 empty))