 reserve U for set,
         X, Y for Subset of U;

theorem Th10:
  for U being non empty set, A being Subset of U holds
    { A } is IntervalSet of U
  proof
    let U be non empty set,
        A be Subset of U;
    Inter (A, A) = { A } by Th8;
    hence thesis by Def2;
  end;
