reserve W,X,Y,Z for set,
  f,g for Function,
  a,x,y,z for set;

theorem Th5:
  Y c= Tarski-Class X implies
  Y,Tarski-Class X are_equipotent or Y in Tarski-Class X
proof
 Tarski-Class X is_Tarski-Class_of X by Def4;
then  Tarski-Class X is Tarski;
  hence thesis;
end;
