How to create nested bookmarks in PDF?

            //Initialize the PDF document
            PdfDocument doc = new PdfDocument();
            //Set page mode
            doc.ViewerPreferences.PageMode = PdfPageMode.UseOutlines;
            //Add top level bookmark
            PdfBookmark pdfBookMark = doc.Bookmarks.Add("Top Book Mark");
            //Add a new page to PDF document
            PdfPage page = doc.Pages.Add();
            //Assign the destination
            pdfBookMark.Destination = new PdfDestination(page);
            //Add new bookmarks and destination
            PdfBookmark pageBookMark1 = pdfBookMark.Add("Page 1");
            pdfBookMark[0].Destination = new PdfDestination(page);
            pageBookMark1.Add("Quarter 1");
            pageBookMark1[0].Destination = new PdfDestination(page, new PointF(10, 200));
            pageBookMark1.Add("Quarter 2");
            pageBookMark1[1].Destination = new PdfDestination(page, new PointF(10, 400));
            pageBookMark1.Add("Quarter 3");
            pageBookMark1[2].Destination = new PdfDestination(page, new PointF(10, 600));
            // Save and close the document.
            doc.Save("Sample.pdf");
            doc.Close(true);

        'Initialize the PDF document
        Dim doc As New PdfDocument()
        'Set page mode
        doc.ViewerPreferences.PageMode = PdfPageMode.UseOutlines
        'Add top level bookmark
        Dim pdfBookMark As PdfBookmark = doc.Bookmarks.Add("Top Book Mark")
        'Add a new page to PDF document
        Dim page As PdfPage = doc.Pages.Add()
        'Assign the destination
        pdfBookMark.Destination = New PdfDestination(page)
        'Add new bookmarks and destination
        Dim pageBookMark1 As PdfBookmark = pdfBookMark.Add("Page 1")
        pdfBookMark(0).Destination = New PdfDestination(page)
        pageBookMark1.Add("Quarter 1")
        pageBookMark1(0).Destination = New PdfDestination(page, New PointF(10, 200))
        pageBookMark1.Add("Quarter 2")
        pageBookMark1(1).Destination = New PdfDestination(page, New PointF(10, 400))
        pageBookMark1.Add("Quarter 3")
        pageBookMark1(2).Destination = New PdfDestination(page, New PointF(10, 600))
        ' Save and close the document.
        doc.Save("Sample.pdf")
        doc.Close(True)