(define rgb.close?
  (lambda (color1 color2)
    (and (>= 16 (abs (- (rgb.red color1) (rgb.red color2))))
         (>= 16 (abs (- (rgb.green color1) (rgb.green color2))))
         (>= 16 (abs (- (rgb.blue color1) (rgb.blue color2)))))))

(define colors.remove-close
  (lambda (colors color)
    (letrec ((kernel
              (lambda (colors color)
                ; I need a base case!
                (if (null? colors)
                    null
                    ; Check if the car is close
                    (if (rgb.close? (car colors) color)
                        (kernel (cdr colors) color)
                        (cons (car colors) (kernel (cdr colors) color)))))))
      
      ; CHECK PRECONDITIONS
      (cond
        ; (1) Make sure that colors is a list
        ((not (list? colors))
         (throw "colors.remove-close: parameter is not a list: " colors))
        ; (2) Make sure that each element in the list is an RGB color
        ((not (all-rgb? colors))
         (throw "colors.remove-close: at least one elemento f the list is not a color"))
        ; (3) Make sure that color is a color
        ((not (rgb? color)) 
         (throw "colors.remove-close: parameter is not an RGB color: " color))
        ; IF ALL THE PRECONDITIONS SUCCEED, RUN THE KERNEL
        (else (kernel colors color)))))) 


(define colors
  (list
   (rgb.new 255 0 0)
   (rgb.new 250 10 10)
   (rgb.new 245 10 10)
   (rgb.new 128 10 10)
   (rgb.new 0 255 255)
   (rgb.new 0 255 0)
   (rgb.new 255 255 255)
   (rgb.new 255 15 0)
   ))

(define all-rgb?
  (lambda (lst)
    (or (null? lst)
        (and (rgb? (car lst))
             (all-rgb? (cdr lst))))))