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

theorem :: Formula (19)
  (X <> {} & Y = {}) or (X = {} & Y <> {}) implies
    (delta_L R).(X,Y) = 1 / 2
  proof
    assume (X <> {} & Y = {}) or (X = {} & Y <> {}); then
    (CMap kappa R).(X,Y) + (CMap kappa R).(Y,X) = 1 by Lemma6f;
    hence thesis by DeltaL;
  end;
