reserve Y for non empty TopStruct;

theorem Th6:
  for A, B being Subset of Y holds A is T_0 or B is T_0 implies A /\ B is T_0
proof
  let A, B be Subset of Y;
  assume
A1: A is T_0 or B is T_0;
  hereby
    per cases by A1;
    suppose
      A is T_0;
      hence thesis by Th5,XBOOLE_1:17;
    end;
    suppose
      B is T_0;
      hence thesis by Th5,XBOOLE_1:17;
    end;
  end;
end;
