reserve X for non empty TopSpace;
reserve x for Point of X;
reserve U1 for Subset of X;

theorem
  for X be non empty TopSpace, A be Subset of X holds [#]X is
  a_neighborhood of A
proof
  let X be non empty TopSpace, A be Subset of X;
  Int [#]X = [#]X by TOPS_1:15;
  hence A c= Int [#]X;
end;
