reserve V for RealLinearSpace;
reserve W,W1,W2,W3 for Subspace of V;
reserve u,u1,u2,v,v1,v2 for VECTOR of V;
reserve a,a1,a2 for Real;
reserve X,Y,x,y,y1,y2 for set;

theorem
  W1 is Subspace of W2 implies W2 + (W1 /\ W3) = (W1 + W2) /\ (W2 + W3)
  by Lm11,RLSUB_1:30;
