anchor.diff:  
   (defstruct (buffer-stream (:constructor %make-buffer-stream))
     vao
  -  (%start 0 :type unsigned-byte) ;; unsigned-byte ≡ (integer 0 *)
  -  (%start-byte (null-pointer) :type foreign-pointer)
  +  (%start 0 :type (unsigned-byte 64))
  +  (%start-byte 0 :type (unsigned-byte 64))
     (length 1 :type unsigned-byte)
     (%index-type nil :type symbol)
     (%index-type-size 0 :type (unsigned-byte 8))
  
positive.code:    :vao vao
     :%start (or start 0)
     :%start-byte (if (%valid-index-type-p index-type)
                      (make-pointer (* start (foreign-type-size index-type)))
                      (null-pointer))
     :length (or length 1)
     :%index-type index-type
     :%index-type-size (if (%valid-index-type-p index-type)
  
negative.code:           (buffer-stream-start stream) (buffer-stream-start stream)))
    value)
  
  (declaim (ftype (function (buffer-stream) (unsigned-byte 64))
                  buffer-stream-start))
  (defun buffer-stream-start (stream)
    (buffer-stream-%start stream))
  
  