 reserve U for set,
         X, Y for Subset of U;
 reserve U for non empty set,
         A, B, C for non empty IntervalSet of U;

theorem
  (Inter ({}U,{}U))^ = Inter ([#]U,[#]U)
  proof
    (Inter ({}U,{}U))^ = Inter ([#]U,({}U)`) by Th46;
    hence thesis;
  end;
