theorem Th12:
  D is Dynkin_System of Omega & D is intersection_stable implies (
  A in D & B in D implies A\B in D)
proof
  assume that
A1: D is Dynkin_System of Omega and
A2: D is intersection_stable;
  assume that
A3: A in D and
A4: B in D;
  B`in D by A1,A4,Def5;
  then A /\ (B`) in D by A2,A3,FINSUB_1:def 2;
  hence thesis by SUBSET_1:13;
end;
