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

theorem
  for U1,U2 being Universe st U1 in U2 holds
  for x being Set of U1 holds x is Set of U2
  proof
    let U1,U2 be Universe;
    assume
A1: U1 in U2;
    let x be Set of U1;
    x is U1-set by Def10;
    then x is U2-set by A1,CLASSES4:def 1;
    hence thesis by Def10;
  end;
