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, Y being a_neighborhood
  of A holds A c= Y
proof
  let X be non empty TopSpace, A be Subset of X, Y be a_neighborhood of A;
  A c= Int Y & Int Y c= Y by Def2,TOPS_1:16;
  hence thesis;
end;
