Problem 4.   You spend your summer consulting for a media conglomerate that owns several radio stations. Much of your code manipulates songs and song play-lists that are represented via the following data definition:

(define-struct song (artist title year))

;;; A Song is (make-song String String Number)

;;; A List of Songs (LOS) is one of:
;;; - empty
;;; - (cons Song LOS)

Your manager needs the function, @noligsoldies, which takes as its input a list of songs, and returns the list containing all the songs recorded before 1975. Design this function.

;;; Grader: Theo

;;; oldies: LOS -> LOS [1pt]
;;; Filter input, keeping songs recorded before 1975. [1pt]
(define (oldies los) 
  ;; [2pt: for two arms of COND.]
  (cond [(empty? los) empty] ; [1pt, for base case]
        ;; [1pt: nested conditional]
        ;; [1pt: natural recursion]
        ;; [1pt: correctness]
        [else (cond [(< (song-year (first los)) 1975)
                     (cons (first los) 
                           (oldies (rest los)))]
                    [else (oldies (rest los))])]))

;;; Examples/tests: [2pt for a non-trivial example or two examples]
(equal? empty (oldies empty))
(equal? (oldies (cons (make-song "Let it bleed" "Rolling Stones" 1969)
                      (cons (make-song "Achy breaky heart" "Billy Ray Cyrus"
        (cons (make-song "Let it bleed" "Rolling Stones" 1969) empty))