Write the procedure new-item!, which consumes the name of an item, its price, and the number of copies that arrived from the warehouse. It extends the current-inventory appropriately. That is, if the item is already in the database, it modifies the corresponding record; otherwise it adds an appropriate record.

Write the procedure inflate!, which consumes a percentage and increases the price of each item in current-inventory accordingly.

Write the procedure discontinued!, which consumes the name of an item and removes the item from the list.

The implementation requires two procedures:

1. init!, which initializes the light to GREEN
2. next!, which switches the traffic light from its current state into the next one. In particular, the sequence

   (begin
     (next!) 
     (next!) 
     (next!))
   

   should cycle the traffic light through all three states back to whatever was its intial state.

Here are the three stages:

They were produced with the following constants:

  ;; ---------------------------------------------------------
  ;; Basic constanst: 
  (define WIDTH  50)
  (define HIGHT 160)
  (define RADIUS 20)

  (define DELTA 10)

  (define X (quotient WIDTH 2))

  ;; off-set : num -> num
  (define (off-set y) (+ y DELTA (* 2 RADIUS)))

  (define Y:RED (off-set (- RADIUS)))
  (define Y:YEL (off-set Y:RED))
  (define Y:GRN (off-set Y:YEL))

  ;; A light is (make-light posn color)
  (define-struct light (center color))

  (define RED:LIGHT (make-light (make-posn X Y:RED) RED))
  (define YEL:LIGHT (make-light (make-posn X Y:YEL) YELLOW))
  (define GRN:LIGHT (make-light (make-posn X Y:GRN) GREEN))

  ;; cycle : -> void
  ;; effect: cycle through the three colors, with one-second breaks
  (define (cycle)
    (next)
    (sleep 1)
    (cycle))

  (init)
  (cycle)