 reserve R for 1-sorted;
 reserve X,Y for Subset of R;
 reserve R for finite 1-sorted;
 reserve X,Y for Subset of R;
 reserve R for finite Approximation_Space;
 reserve X,Y,Z,W for Subset of R;

theorem Prop2c: :: Proposition 2 c)
  X <> {} implies kappa (X,{}R) = 0
  proof
    assume X <> {}; then
    kappa (X,{}R) = card (X /\ {}R) / card X by KappaDef;
    hence thesis;
  end;
