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Here is some alternative text, with a bit more explanation.

\begin{memberdesc}{unused_data}
A string which contains any bytes past the end of the
compressed data.
That is, this remains \code{""} until the last byte that
contains compression data is available.  If the whole string
turned
out to contain compressed data, this is \code{""}, the empty
string. 

The only way to determine where a string of compressed data
ends is by
actually decompressing it.  This means that when compressed
data is
contained part of a larger file, you can only find the end
of it by
reading data and feeding it followed by some non-empty
string into a 
decompression object's \method{decompress} method until the 
\member{unused_data} attribute is no longer the empty string.  
\end{memberdesc}

\begin{memberdesc}{unconsumed_tail}
A string that contains any data that was not consumed by the
last
\method{decompress} call because it exceeded the limit for the
uncompressed data buffer.  This data has not yet been seen
by the 
zlib machinery, so you must feed it (possibly with further data 
concatenated to it) back to a subsequent \method{decompress}
method 
call in order to get correct output.
\end{memberdesc}