 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_L R).(X,Y) = (delta_L R).(Y,X)
  proof
    (delta_L R).(X,Y) = ((CMap kappa R).(X,Y) + (CMap kappa R).(Y,X)) / 2
      by DeltaL;
    hence thesis by DeltaL;
  end;
