theorem Th38:
  P.(A \/ B) = P.A + P.B - P.(A /\ B)
proof
  P.(A \/ B) = P.A + P.(B \ (A /\ B)) by Th37
    .= P.A + (P.B - P.(A /\ B)) by Th33,XBOOLE_1:17;
  hence thesis;
end;
