theorem Th24:
  for E being finite non empty set, A,B being Event of E st B c= A
  holds prob(A \ B) = prob(A) - prob(B)
proof
  let E be finite non empty set, A,B be Event of E;
  assume B c= A;
  then prob(A /\ B) = prob(B) by XBOOLE_1:28;
  hence thesis by Th23;
end;
