reserve U for Universe;
reserve x for Element of U;

theorem
  for U1,U2 being Universe st U1 in U2 holds
  for x being object st x is U1-Class holds x is U2-set
  proof
    let U1,U2 be Universe;
    assume
A1: U1 in U2;
    let x be object;
    assume
A2: x is U1-Class;
    bool U1 in U2 by A1,CLASSES4:def 3;
    hence thesis by A2,CLASSES4:def 1;
  end;
