anchor.diff:                  for cx fixnum = (minus xd dx)
                   for cy fixnum = (minus yd dy) do
                   (loop for i fixnum from 0 below ncomp do
  -                      (setf (u8ref (aref outbuf i) cx cy)
  +                      (setf (u16ref (aref outbuf i) cx cy)
                               (minus (aref inbuf (plus pos i)) 128)))))
       (values xend yend)))
   
  
positive.code: ;;  -*- Mode: LISP; Package: (JPEG :use (common-lisp)) -*-
  ;;; Generic Common Lisp JPEG encoder/decoder implementation
  ;;; Copyright [c] 1999,2015,2016 Eugene Zaikonnikov <eugene@funcall.org>
  ;;;               
  ;;; This software is distributed under the terms of BSD-like license
  
negative.code: 
  (defmacro s16ref (data x y)
    `(the sint16 (aref (the sint16-array (aref (the sint16-2d-array ,data) ,y)) ,x)))
  
  (defmacro u16ref (data x y)
    `(the uint16 (aref (the uint16-array (aref (the uint16-2d-array ,data) ,y)) ,x)))
  
  (defmacro fixref (data x y)
    `(the fixnum (aref (the fixnum-array (aref (the fixnum-2d-array ,data) ,y)) ,x)))
  