anchor.diff:  
   (defgeneric render-scanline (ppu)
     (:method ((ppu ppu)) ;; TODO: Mirroring. Scrolling?
  -    (let* ((bd-index (logand (read-vram ppu #x3f00) #x3f))
  -           (bd-color (get-color bd-index)))
  +    (let ((bd-color (get-color ppu #x3f00)))
         (dotimes (x +width+)
           (let* ((bg-color (when (show-bg ppu)
                              (get-bg-pixel ppu x)))
  
positive.code: 
  ;;;; Colors
  
  (declaim (inline make-color get-color put-pixel))
  (defstruct color
    (r 0 :type u8)
    (g 0 :type u8)
    (b 0 :type u8))
  
  (defun get-color (index)
    (let* ((base (* index 3))
           (red   (aref *color-palette* (+ 2 base)))
           (green (aref *color-palette* (+ 1 base)))
           (blue  (aref *color-palette* (+ 0 base))))
  
negative.code:           (aref *frame* (+ base 1)) (color-g color)
            (aref *frame* (+ base 2)) (color-b color))))
  
  (defmethod get-pattern-color ((ppu ppu) kind tile x y)
    (let ((offset (+ y (ash tile 4))))
      (case kind
        (:bg (incf offset (bg-pattern-addr ppu)))
  