reserve T for TopSpace,
  A, B for Subset of T;

theorem Th14: :: Remark 2
  A is regular_closed implies A` is regular_open
proof
  assume A is regular_closed;
  then Cl Int A = A by TOPS_1:def 7;
  then Int (Int A)` = A` by TDLAT_3:3;
  then Int Cl(A`) = A` by TDLAT_3:2;
  hence thesis by TOPS_1:def 8;
end;
