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

theorem Th15: :: Remark 3
  Int Cl A is regular_open & Cl Int A is regular_closed
proof
A1: Cl Int Cl Int A = Cl Int A by TOPS_1:26;
  Int Cl Int Cl A = Int Cl A by TDLAT_1:5;
  hence thesis by A1,TOPS_1:def 7,def 8;
end;
