reserve n,m,k for Element of NAT,
  x,X for set,
  A1 for SetSequence of X,
  Si for SigmaField of X,
  XSeq for SetSequence of Si;
reserve Omega for non empty set,
  Sigma for SigmaField of Omega,
  ASeq for SetSequence of Sigma,
  P for Probability of Sigma;

theorem Th5:
  for A,B being Event of Sigma holds P.(A \ B) = P.(A \/ B) - P.B
proof
  let A,B be Event of Sigma;
  P.(A \/ B) - P.B = P.B + P.(A \ B) - P.B by PROB_1:36;
  hence thesis;
end;
