theorem
  for W1,W2 being strict Subspace of M holds W1 + W2 = W2 iff W1 /\ W2 = W1
proof
  let W1,W2 be strict Subspace of M;
  W1 + W2 = W2 iff W1 is Subspace of W2 by Th8;
  hence thesis by Th16;
end;
