What compression use a sws worksheet file?

edit

It's a tar.bz2 file. You can decompress and unpack a worksheet like so:

tar xvjf Interval_Notation.sws

results in a directory called sage_worksheet with contents:

drwxr-xr-x  7 ?  staff   238 Jun 21 10:15 cells
-rw-r--r--  1 ?  staff  6032 Feb 21  2011 worksheet.html
-rw-------  1 ?  staff  6062 Feb 21  2011 worksheet.txt
-rw-r--r--  1 ?  staff   169 Feb 21  2011 worksheet_conf.pickle

You can also use the unix file command to investigate if you don't know.

% file zemek.sws 
zemek.sws: bzip2 compressed data, block size = 900k

It's a bzip2'ed file - apparently a tar.bz2 file, to be precise. See http://ask.sagemath.org/question/961/large-worksheet-upload-to-notebook, among others.