title: Xml searching node issue
body: <p>I have a following XML structure :</p>
  
  <pre><code>&lt;html xmlns ="http://www.w3.org/1999/xhtml" &gt;
   &lt;head&gt;
    . .
   &lt;/head&gt;
   &lt;body&gt;
    &lt;div id="1"&gt;&lt;/div&gt;
    &lt;div id="2"&gt;&lt;/div&gt;
   &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>I used linq to xml to access div with id = "2". I loaded document in XDocument :</p>
  
  <pre><code>   XDocument  ndoc = XDcoument.load(path);
     XElement n = new XElement("name","value");
     XNamespace xn = "http://www.w3.org/1999/xhtml";
  
  ndoc.Descendants(xn + "div").Single(p =&gt; p.Attribute("id").Value == "1").Add(n);
  
                               OR
  
  ndoc.Descendants("div").Single(p =&gt; p.Attribute("id").Value == "1").Add(n);
  </code></pre>
  
  <p>I tried out both case in each case there was an exception sequence contain no element. Whats the problem here?</p>