theorem Th6:
  for A,B being Event of Sigma st A c= B & P.B = 0 holds P.A = 0
proof
  let A,B be Event of Sigma;
  assume A c= B & P.B = 0;
  then P.A <= 0 by PROB_1:34;
  hence thesis by PROB_1:def 8;
end;
