anchor.diff:    (the fixnum (+ (the fixnum (* nx jy)) jx)))
   
   (defun image-value (f i j nx fmax fmin)
  -  (let ((fc (mem-aref f (index nx i j))))
  +  (let ((fc (memory-block-aref f (index nx i j))))
       (truncate (* 256.0
                    (/ (- fc fmin) (- fmax fmin))))))
   
  
positive.code: 
  (in-package :cl-user)
  (defpackage cl-cuda-examples.diffuse0
    (:use :cl
          :cl-cuda)
    (:shadow :start-timer
             :stop-timer)
    (:export :main))
  (in-package :cl-cuda-examples.diffuse0)
  
  
negative.code:         (let ((j (index nx jx jy))
                (x (- (* dx (+ (float jx 1.0) 0.5)) 0.5))
                (y (- (* dy (+ (float jy 1.0) 0.5)) 0.5)))
            (setf (memory-block-aref f j)
                  (exp (* (- alpha)
                          (+ (* x x) (* y y)))))))))
    (sync-memory-block f :host-to-device))
  
  (defvar +block-dim-x+ 256)
  (defvar +block-dim-y+ 1)
  