theorem Th36:
  P.(A \/ B) = P.A + P.(B \ A)
proof
A1: A misses (B \ A) by XBOOLE_1:79;
  P.(A \/ B) = P.(A \/ (B \ A)) by XBOOLE_1:39
    .= P.A + P.(B \ A) by A1,Def8;
  hence thesis;
end;
