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function UnCheckIndex(FileDbf: string): Boolean;

 var

   Dbf: file;

   Car: Char;

 begin

   Result := T;

   AssignFile(Dbf, FileDbf);

   Car := #0;

   {$I-}

   Reset(Dbf, 1);

   if not ErrorIO(FileDbf, IoResult) then

    begin

     Seek(Dbf, 28);

     {Flag's position}

     if not ErrorIO(FileDbf, IoResult) then

       BlockWrite(Dbf, Car, 1, Num_R)

     else

       Result := F;

     CloseFile(Dbf);

     if ErrorIO(FileDbf, IoResult) then

       Result := F;

   end

   else

     Result := F;

   {$I+}

 end;

procedure TForm1.Button1Click(Sender: TObject);

 begin

   if UnCheckIndex('MyBase.dbf') then

     ShowMessage('Flag removed');

 end;