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 Th98;
    hence thesis by Th106;
  end;
