anchor.diff:  	     (let ((byte2 (read-byte stream)))
   	       (case byte2
   		 ((#.(char-code #\1)) (read-pbm-p1 stream))
  -		 ((#.(char-code #\4)) (read-pbm-p4 stream)) 
  +		 ((#.(char-code #\4)) (read-pbm-p4 stream))
   		 ((#.(char-code #\2)) (read-pgm-p2 stream))
   		 ((#.(char-code #\5)) (read-pgm-p5 stream))
   		 ((#.(char-code #\3)) (read-ppm-p3 stream)) ; ASCII
  
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: 
  (defmethod image-height ((image image))
    (car (array-dimensions (image-pixels image))))
  
  (defmethod image-pixel ((image image) x y)
    (aref (image-pixels image) y x))
  
  