ormat FOO that
  encapsulates a DEFLATE data stream but adds four signature octets,
  F0 0D 00 D1, to the start of the output data stream, and adds a
  trailing 32-bit length value, MSB first, after the end. It could be
  implemented like this:

<pre>
(defclass foo-compressor (deflate-compressor)
  ((data-length
    :initarg :data-length
    :accessor data-length))
  (:default-initargs
   :data-length 0))

(defmethod <a href='#start-data-format'>start-data-format</a> :before ((compressor foo-compressor))
  