reserve T for non empty TopSpace,
  A, B for Subset of T,
  F, G for Subset-Family of T;

theorem
  for A being Subset of T st A is open holds A is G_delta
proof
  let A be Subset of T;
  assume A is open;
  then reconsider F = {A} as open countable Subset-Family of T by Th19;
  take F;
  thus thesis by SETFAM_1:10;
end;
