 reserve R for finite Approximation_Space;
 reserve X,Y,Z for Subset of R;
 reserve kap for RIF of R;

theorem :: Proposition 7 bL)
  (delta_1 R).(X,Y) = (delta_1 R).(Y,X)
  proof
    (delta_1 R).(X,Y) = (CMap kappa_1 R).(X,Y) + (CMap kappa_1 R).(Y,X)
      by Delta1;
    hence thesis by Delta1;
  end;
