title: how to generate unique string
body: <p>I want to transform an XML document into HTML. Some XML elements have links to others documents like:</p>
  
  <pre><code>&lt;link href="1.html"&gt;
  </code></pre>
  
  <p>In the HTML output, I want to get:</p>
  
  <pre><code>&lt;a href="1.html&amp;no_cache={unique_id}"&gt;
  </code></pre>
  
  <p>How can I generate this unique fairly large ID?</p>