anchor.diff:  ;;;; Colors
   
   (declaim (inline get-color))
  -(defmethod get-color ((ppu ppu) vram-index)
  +(defun get-color (ppu vram-index)
     (let* ((color-index (logand (read-vram ppu vram-index) #x3f))
            (base (* color-index 3))
            (red   (aref *color-palette* (+ 2 base)))
  
positive.code: ;;;; Sprite RAM/Object Attribute Memory (OAM)
  
  (declaim (inline read-oam))
  (defmethod read-oam ((ppu ppu) addr)
    (aref (ppu-oam ppu) addr))
  
  (defmethod write-oam ((ppu ppu) val)
    (with-accessors ((addr ppu-oam-addr)) ppu
      (setf (aref (ppu-oam ppu) addr) val)
      (incf addr)))
  
negative.code:       :above
        :below))
  
  (defun on-scanline (sprite ppu)
    (with-accessors ((scanline ppu-scanline)) ppu
      (if (< scanline (sprite-y sprite))
          nil
  