private static void Plain(StringBuilder builder, ref ToPlainTextState state, IEnumerable <XNode> nodes)
        {
            foreach (var node in nodes)
            {
                if (node is XElement)
                {
                    var element = (XElement)node;
                    var tag     = element.Name.LocalName.ToLower();

                    if (tag == "br")
                    {
                        builder.AppendLine();
                        state = ToPlainTextState.StartLine;
                    }
                    else if (NonVisibleTags.Contains(tag))
                    {
                    }
                    else if (InlineTags.Contains(tag))
                    {
                        Plain(builder, ref state, element.Nodes());
                    }
                    else
                    {
                        if (state != ToPlainTextState.StartLine)
                        {
                            builder.AppendLine();
                            state = ToPlainTextState.StartLine;
                        }

                        Plain(builder, ref state, element.Nodes());
                        if (state != ToPlainTextState.StartLine)
                        {
                            builder.AppendLine();
                            state = ToPlainTextState.StartLine;
                        }
                    }
                }
                else if (node is XText)
                {
                    var text = (XText)node;
                    Process(builder, ref state, text.Value.ToCharArray());
                }
            }
        }
        private static void Plain(StringBuilder builder, ref ToPlainTextState state, IEnumerable <HtmlAgilityPack.HtmlNode> nodes)
        {
            foreach (var node in nodes)
            {
                if (node is HtmlAgilityPack.HtmlTextNode)
                {
                    var text = (HtmlAgilityPack.HtmlTextNode)node;
                    Process(builder, ref state, HtmlAgilityPack.HtmlEntity.DeEntitize(text.Text).ToCharArray());
                }
                else
                {
                    var tag = node.Name.ToLower();

                    if (tag == "br")
                    {
                        builder.AppendLine();
                        state = ToPlainTextState.StartLine;
                    }
                    else if (NonVisibleTags.Contains(tag))
                    {
                    }
                    else if (InlineTags.Contains(tag))
                    {
                        Plain(builder, ref state, node.ChildNodes);
                    }
                    else
                    {
                        if (state != ToPlainTextState.StartLine)
                        {
                            builder.AppendLine();
                            state = ToPlainTextState.StartLine;
                        }

                        Plain(builder, ref state, node.ChildNodes);
                        if (state != ToPlainTextState.StartLine)
                        {
                            builder.AppendLine();
                            state = ToPlainTextState.StartLine;
                        }
                    }
                }
            }
        }