theorem Th21:
  (a-reflection).b - b = 2 * (a - b)
  proof
    2*a - b - b = 2*a - (b + b) by RLVECT_1:27
    .= 2*a - 2*b by Th3
    .= 2*(a-b) by RLVECT_1:34;
    hence thesis by Def4;
  end;
