add union,unions, and sempty to SetLike class, remove 'Monoid' superclass
add union,unions, and sempty to SetLike class, remove 'Monoid' superclass

darcs-hash:20070606073202-1a7c6-7322889cfbfa04b974786dfee51580b5a1d51716

jimcrayne/jhc,jimcrayne/jhc,jimcrayne/jhc,jimcrayne/jhc