(with-output-to-string (s)
  (flet ((add (start end)
           (loop for offset from (+ start i) to (+ end i)
                 for octet = (aref cluster offset)
                 when (and (/= octet 0)
                           (/= octet 255))
                 do (write-char (code-char octet) s))))
    (add 1 10)
    (add 14 25)
    (add 28 31)))