(write-sequence (map 'vector #'char-code (format nil "P6~%")) str)
(write-sequence (map 'vector #'char-code (format nil "143 143~%")) str)
(write-sequence (map 'vector #'char-code (format nil "255~%")) str)
(write-sequence (map 'vector #'char-code (format nil "P6~%")) str)
(write-sequence (map 'vector #'char-code (format nil "143 143~%")) str)
(write-sequence (map 'vector #'char-code (format nil "255~%")) str)