theorem
  for V be Abelian add-associative right_zeroed right_complementable
  non empty addLoopStr, S,T be finite Subset of V holds Sum(T /\ S) = Sum(T) +
  Sum(S) - Sum(T \/ S)
proof
  let V be Abelian add-associative right_zeroed right_complementable non
  empty addLoopStr, S,T be finite Subset of V;
  Sum(T \/ S) = Sum(T) + Sum(S) - Sum(T /\ S) by Th13;
  then Sum(T) + Sum(S) = Sum(T /\ S) + Sum(T \/ S) by RLSUB_2:61;
  hence thesis by RLSUB_2:61;
end;
