theorem
  Sum(T /\ S) = Sum(T) + Sum(S) - Sum(T \/ S)
proof
  Sum(T \/ S) = Sum(T) + Sum(S) - Sum(T /\ S) by Th10;
  then Sum(T) + Sum(S) = Sum(T /\ S) + Sum(T \/ S) by RLSUB_2:61;
  hence thesis by RLSUB_2:61;
end;
