theorem Th13:
  Sum(T \ S) = Sum(T) - Sum(T /\ S)
proof
  T \ (T /\ S) = T \ S by XBOOLE_1:47;
  then Sum(T \ S) = Sum(T \/ (T /\ S)) - Sum(T /\ S) by Th12;
  hence thesis by XBOOLE_1:22;
end;
