title: How to change a attribute conditionally be checking its sibling's value?
body: <p>I want to change some attribute values only for some specific element.
  For example, is it possible to change @lang only for the books whose price is 29.99 in below XML?</p>
  
  <pre><code>&lt;book&gt;
    &lt;title lang="fr"&gt;Harry Potter&lt;/title&gt;
    &lt;price&gt;29.99&lt;/price&gt;
  &lt;/book&gt;
  
  &lt;book&gt;
    &lt;title lang="eng"&gt;Learning XML&lt;/title&gt;
    &lt;price&gt;39.95&lt;/price&gt;
  &lt;/book&gt;
  </code></pre>