XML LINQ C#

using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using System.Xml;
using System.Xml.Linq;
public class MainClass {
    public static void Main() {
        XElement firstParticipant;
        XDocument xDocument = new XDocument(
         new XDeclaration("1.0", "UTF-8", "yes"),
         new XDocumentType("Books", null, "Books.dtd", null),
         new XProcessingInstruction("Book", "out-of-print"),
       new XElement("Books",
       new XComment("Begin Of List"), firstParticipant =
       new XElement("Book",
       new XAttribute("type", "Author"),
       new XElement("FirstName", "J"),
       new XElement("LastName", "R")),
       new XElement("Book",
       new XAttribute("type", "Author"),
       new XElement("FirstName", "E"),
       new XElement("LastName", "B")),
       new XComment("End Of List")));
        foreach (XNode node in firstParticipant.NodesAfterSelf()) {
            Console.WriteLine(node);
        }
    }
}