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