theorem LemmaProp2b: :: Proposition 2 b) from left to right, without X <> {}
  kappa (X,Y) = 0 implies X misses Y
  proof
     assume
A1:  kappa (X,Y) = 0;
     per cases;
     suppose
A2:    X <> {}; then
       kappa (X,Y) = card (X /\ Y) / card X by KappaDef;
       hence thesis by A1,A2;
     end;
     suppose X = {};
       hence thesis;
     end;
   end;
