title: xmlstarlet not working for simple selection based on child string value
body: <p>I am working on XML file like</p>
  
  <pre><code>&lt;bookstore&gt; 
     &lt;book category="cooking"&gt; 
         &lt;title lang="en"&gt;Everyday Italian&lt;/title&gt; 
         &lt;author&gt;adc&lt;/author&gt; 
         &lt;year&gt;2005&lt;/year&gt; 
         &lt;price&gt;30.00&lt;/price&gt; &lt;/book&gt; 
     &lt;book category="children"&gt; 
         &lt;title lang="en"&gt;Harry Potter&lt;/title&gt; 
         &lt;author&gt;xyz&lt;/author&gt; 
         &lt;year&gt;2005&lt;/year&gt; 
         &lt;price&gt;29.99&lt;/price&gt; &lt;/book&gt; 
  &lt;/bookstore&gt;
  </code></pre>
  
  <p>Using xmlstarlet on linux </p>
  
  <pre><code>xmlstarlet sel -t -m 'bookstore/author [1]' -n books.xml 
  </code></pre>
  
  <p>Would give output as <code>"adc"</code>, but how can I find the field when I already know the value?</p>
  
  <p>Like I know author tag has value <code>"adc"</code>. 
  How should I find <code>bookstore/author [x]</code> the <code>x</code>?</p>