reserve X for set;
reserve UN for Universe;

theorem Th73:
  for X being set
  for U1 being Universe st U1 in Tarski-Class X
  holds Tarski-Class U1 c= Tarski-Class X
  proof
    let X be set;
    let U1 be Universe;
    assume U1 in Tarski-Class X;
    then Tarski-Class X is_Tarski-Class_of U1;
    hence thesis by CLASSES1:def 4;
  end;
