 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 LemmaProp4a:
  kappa_1 (X,Y) = 0 implies Y = {}
  proof
    assume kappa_1 (X,Y) = 0; then
    card Y / card (X \/ Y) = 0 by Kappa1;
    hence thesis;
  end;
