reserve X for set;

theorem
  for S being Field_Subset of X, M being Measure of S holds {} in S & X
in S & for A,B being set st A in S & B in S holds X \ A in S & A \/ B in S & A
  /\ B in S by Def1,FINSUB_1:def 1,def 2,PROB_1:4,5;
