anchor.diff:  (defmacro with-write-pnm-loop ((magic-number max-value) &body body)
     `(let ((height (car (array-dimensions picture)))
   	 (width (cadr (array-dimensions picture))))
  -     (format stream "P~A~%" ,magic-number)	    
  +     (format stream "P~A~%" ,magic-number)
        (format stream "~A ~A~%" width height)
        (when ,max-value
          (format stream "~A~%" ,max-value))
  
positive.code: (defpackage "IMAGE"
  ; (:use #:clim-lisp)
    (:use #:clim-clx #:common-lisp)
    (:export    
     "WRITE-PNM" "READ-IMAGE-FILE"
     "IMAGE" "IMAGE-COLOR" "IMAGE-GADGET" "IMAGE-HEIGHT"
     "IMAGE-PIXEL" "IMAGE-PIXELS" "IMAGE-WIDTH"
  
negative.code: (defun write-ppm-p3 (stream picture)
    (with-write-pnm-loop (3 255)
      (let ((rgb (aref picture r c)))
        (format stream "~A ~A ~A~%"
  	      (red-component rgb)
  	      (green-component rgb)
  	      (blue-component rgb)))))
  
  