title: Add new node to xml tree
body: <p>I am generating a xml tree:</p>
  
  <pre><code>var root = new XElement("Root");
  for(int i =0;i&lt;3;i++)
  { 
     var sub0lvl = new XElement(String.Format("sub{0}",i));
     root.Add(sub0lvl);
     for(int j=0;j&lt;2;j++)
     {
       sub0lvl.Add(new XElement(String.Format("subsub{0}",i)));
     }
  }
  </code></pre>
  
  <p>This code generate follow xml tree:</p>
  
  <pre><code>&lt;Root&gt;
    &lt;sub0&gt;
      &lt;subsub0 /&gt;
      &lt;subsub0 /&gt;
    &lt;/sub0&gt;
    &lt;sub1&gt;
      &lt;subsub1 /&gt;
      &lt;subsub1 /&gt;
    &lt;/sub1&gt;
    &lt;sub2&gt;
      &lt;subsub2 /&gt;
      &lt;subsub2 /&gt;
    &lt;/sub2&gt;
    &lt;sub8&gt;
      &lt;subsub123 /&gt;
    &lt;/sub8&gt;
  &lt;/Root&gt;
  </code></pre>
  
  <p>And i want to add new element to sub1 node using code like that </p>
  
  <pre><code>root.Add(new XElement("sub1",new XElement("subsub123")));
  </code></pre>
  
  <p>But this code is not work as i wish. It's just add new same node to root. What is the right way to do that?</p>