title: How to multiply values in XPATH?
body: <p>Suppose I have a bunch of</p>
  
  <pre><code>&lt;item price='n' sold='m'&gt;
  ...
  &lt;/item&gt;
  </code></pre>
  
  <p>I want to do price * sold on each of the items. I am trying </p>
  
  <pre><code>'//item/(@price * @sold)' 
  </code></pre>
  
  <p>from inside lxml in python, but I keep getting the error <em>'Invalid Expression'</em>. The same happens with xmllint command too. 
  But the same query works in Oxygen xml editor. </p>
  
  <p>How do I make this work? </p>