Changes by: Xin Yu (xiny at cs.caltech.edu)
Date: 2003-02-21 18:07:07 -0800 (Fri, 21 Feb 2003)
Revision: 4107
Log message:
1. Added the fold form of rewritings.
2. Added the commutative property and defined commutative semigroup/monoid
and abelian group.