theorem Th16:
  W1 /\ W2 is Subspace of W1 & W1 /\ W2 is Subspace of W2
proof
  the carrier of W1 /\ W2 c= the carrier of W1 by Lm4;
  hence W1 /\ W2 is Subspace of W1 by RLSUB_1:28;
  the carrier of W2 /\ W1 c= the carrier of W2 by Lm4;
  then the carrier of W1 /\ W2 c= the carrier of W2 by Th14;
  hence thesis by RLSUB_1:28;
end;
