> <i>compressor</i> => <i>callback</i><br>
(<tt>setf</tt> (<b>callback</b> <i>compressor</i>) <i>new-value</i>) 
=> <i>new-value</i>

<blockquote>
Gets or sets the callback function of <i>compressor</i>. The callback
should be a function of two arguments, an octet vector and an end
index, and it should process all octets from the start of the vector
below the end index as the compressed output data stream of the
compressor. See <a href='#make-stream-output-callback'><tt>MAKE-STREAM-OUTPUT-CALLBACK</tt></a>
