 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 Th44:
  for U being non empty set holds Inter ({}U,{}U) is non empty IntervalSet of U
  proof
    let U be non empty set;
    Inter ({}U,{}U) = {{}} by Th8;
    hence thesis;
  end;
