Tue, 22 Mar 2011 16:44:57 +0100 hoelzl add ring_of_sets and subset_class as basis for algebra
(0) -30000 -10000 -3000 -1000 -300 -100 -30 -10 -1 +1 +10 +30 +100 +300 +1000 +3000 +10000 +30000 tip