(The python world seems to think that xml parsing functions are part
of core Python and need not be mentioned. Perhaps pkgsrc's way of
having py-expat is unusual and a bit of DESCR spiffage is in order,
but I'm not sure what's right.)
If your source tree is coming from darcs (i.e. it is in a darcs
repository), this tool will determine the most recent release tag,
count the patches that have been applied since then, and compute a
version number to be written into _version.py.
