anchor.diff:  
   (defun get-bg-pixel (ppu x)
     (let* ((x (+ (getf (ppu-meta ppu) :x) x))
  -         (y (+ (getf (ppu-meta ppu) :y) (getf (ppu-meta ppu) :scanline)))
  +         (y (+ (getf (ppu-meta ppu) :y) (ppu-scanline ppu)))
            (base (nametable-addr (round x 8) (round y 8))) ; TODO: Use floor instead?
            (tile (read-vram ppu (apply '+ (* 32 (first base)) (rest base))))
            (color (get-pixel-color ppu 'foo tile (mod x 8) (mod y 8))))
  
positive.code:     (incf (ppu-addr ppu) (vram-step ppu))
      (if (< addr #x3f00)
          (prog1
              (getf (ppu-meta ppu) :buffer)
            (setf (getf (ppu-meta ppu) :buffer) result))
          result)))
  
  ;;;; Misc Helpers
  
negative.code:       :below))
  
  (defmethod on-scanline ((sprite sprite) (ppu ppu))
    (with-accessors ((scanline ppu-scanline)) ppu
      (if (< scanline (sprite-y sprite))
          nil
          (ecase (sprite-size ppu)
  