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

theorem Th3:
  Y in Tarski-Class X & Z c= Y implies Z in Tarski-Class X
proof
 Tarski-Class X is_Tarski-Class_of X by Def4;
then  Tarski-Class X is Tarski;
then  Tarski-Class X is subset-closed;
  hence thesis;
end;
