theorem
  for W1,W2 being strict Submodule of V holds W1 + W2 = W2 iff W1 /\ W2 = W1
proof
  let W1,W2 be strict Submodule of V;
  W1 + W2 = W2 iff W1 is Submodule of W2 by Th8;
  hence thesis by Th17;
end;
