anchor.diff:  
   (defmethod image-height ((image image))
     (car (array-dimensions (image-pixels image))))
  -   
  +
   (defmethod image-pixel ((image image) x y)
     (aref (image-pixels image) y x))
   
  
positive.code: 
  (defun read-pgm-p5 (stream)
    (with-pnm-header t
      (loop with size of-type fixnum = (* width height) 
  	  with result = (make-array `(,height ,width) :element-type '(unsigned-byte 8))
  	  with vec = (make-array `(,size)
  				 :element-type '(unsigned-byte 8)
  
negative.code: (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 "~A ~A~%" width height)
       (when ,max-value
         (format stream "~A~%" ,max-value))
  