title: Insert a node for an element in XML with Python/ElementTree
body: <p>I need to traverse the XML tree to add sub element when the value is less than 5. 
  For example, this XML can be modified into</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;A value="45"&gt;
      &lt;B value="30"&gt;
          &lt;C value="10"/&gt;
          &lt;C value ="20"/&gt;
      &lt;/B&gt;
      &lt;B value="15"&gt;
          &lt;C value = "5" /&gt;
          &lt;C value = "10" /&gt;
      &lt;/B&gt;
  &lt;/A&gt;
  </code></pre>
  
  <p>this XML.</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;A value="45"&gt;
      &lt;B value="30"&gt;
          &lt;C value="10"/&gt;               
          &lt;C value ="20"/&gt;
      &lt;/B&gt;
      &lt;B value="15"&gt;
          &lt;C value = "5"&gt;&lt;D name="error"/&gt;&lt;/C&gt;
          &lt;C value = "10" /&gt;
      &lt;/B&gt;
  &lt;/A&gt;
  </code></pre>
  
  <p>How can I do that with Python's ElementTree?</p>