Sets, Cwm, and Python 2.3

In order to implement an internal Set type in Cwm (I guess equivilent to 
an rdf bag), as well as for a number of performance improvements in a 
number of different areas, it was suggested to make cwm start using the 
Python 2.3 type ``Set''. Note that this would force all users of cwm to 
be running Python 2.3.

If you have anything against a Python>=2.3 dependency, now is the time 
to mention it, before any code using Set is written.

Yosi