 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 <> {} implies
    (delta_L R).(X,Y) = ((card (X \ Y) / card X) + (card (Y \ X) / card Y)) / 2
  proof
    assume
A1: X <> {} & Y <> {}; then
A2: (CMap kappa R).(X,Y) = card (X \ Y) / card X by PropEx3k;
    (CMap kappa R).(Y,X) = card (Y \ X) / card Y by A1,PropEx3k;
    hence thesis by DeltaL,A2;
  end;
