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: (deftype sint16-array () '(simple-array sint16 (*)))
  (deftype sint16-2d-array () '(simple-array sint16-array (*)))
  
  (deftype fixnum-array () '(simple-array fixnum (*)))
  (deftype fixnum-2d-array () '(simple-array fixnum-array (*)))
  
  
negative.code:     (declare #.*optimize*
               (type fixnum dx dy h w height width ncomp xend yend)
  	     (type uint8-array inbuf)
  	     (type (simple-array uint16-2d-array (*)) outbuf))
      (setf xend (min xend (1- w)))
      (setf yend (min yend (1- h)))
      (loop for yd fixnum from dy to yend
  