Yes, I have inplemented all the shortcut assignment operators.  I also allow added elements as you suggested (with +=) or you can even do:  s << e1 << e2...
[I may have gone a little overboard with the operators].
 
I like your suggestion for membership testing using <= and >=.  What does everyone else think?
 
I have been using + for union and * for intersection.  You think | and & would be better?  in addition too?