iTextSharp.text.pdf.PdfVisibilityExpression.Add C# (CSharp) Метод

        public override bool Add(PdfObject obj)
        {
            if (obj is PdfLayer)
                return base.Add(((PdfLayer)obj).Ref);
            if (obj is PdfVisibilityExpression)
                return base.Add(obj);
            throw new ArgumentException(MessageLocalization.GetComposedMessage("illegal.ve.value"));
        }

        // ===========================================================================
        public void Write(Stream stream)
        {
            // step 1
              using (Document document = new Document()) {
            // step 2
            PdfWriter writer = PdfWriter.GetInstance(document, stream);
            writer.PdfVersion = PdfWriter.VERSION_1_6;
            // step 3
            document.Open();
            // step 4
            PdfContentByte cb = writer.DirectContent;

            PdfLayer dog = new PdfLayer("layer 1", writer);
            PdfLayer tiger = new PdfLayer("layer 2", writer);
            PdfLayer lion = new PdfLayer("layer 3", writer);
            PdfLayerMembership cat = new PdfLayerMembership(writer);
            PdfVisibilityExpression ve1 = new PdfVisibilityExpression(
              PdfVisibilityExpression.OR
            );
            ve1.Add(tiger);
            ve1.Add(lion);
            cat.VisibilityExpression = ve1;
            PdfLayerMembership no_cat = new PdfLayerMembership(writer);
            PdfVisibilityExpression ve2 = new PdfVisibilityExpression(
              PdfVisibilityExpression.NOT
            );
            ve2.Add(ve1);
            no_cat.VisibilityExpression = ve2;
            cb.BeginLayer(dog);
            ColumnText.ShowTextAligned(cb, Element.ALIGN_LEFT, new Phrase("dog"),
                50, 775, 0);
            cb.EndLayer();
            cb.BeginLayer(tiger);
            ColumnText.ShowTextAligned(cb, Element.ALIGN_LEFT, new Phrase("tiger"),
                50, 750, 0);
            cb.EndLayer();
            cb.BeginLayer(lion);
            ColumnText.ShowTextAligned(cb, Element.ALIGN_LEFT, new Phrase("lion"),
                50, 725, 0);
            cb.EndLayer();
            cb.BeginLayer(cat);
            ColumnText.ShowTextAligned(cb, Element.ALIGN_LEFT, new Phrase("cat"),
                50, 700, 0);
            cb.EndLayer();
            cb.BeginLayer(no_cat);
            ColumnText.ShowTextAligned(cb, Element.ALIGN_LEFT,
                new Phrase("no cat"), 50, 700, 0);
            cb.EndLayer();
              }
        }