title: Zebra striping groups of li elements
body: <p>I have the following HTML and I'd like to zebra stripe the contents in groups of 3:</p>
  
  <pre><code>&lt;ul id="item-order"&gt;
  
      &lt;li class="thumbnail"&gt;Item 1&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 2&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 3&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 4&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 5&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 6&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 7&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 8&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 9&lt;/li&gt;
  
  &lt;/ul&gt;
  </code></pre>
  
  <p>So I'd like to generate the following using jQuery:</p>
  
  <pre><code>&lt;ul id="item-order"&gt;
      &lt;li class="thumbnail stripe"&gt;Item 1&lt;/li&gt;
      &lt;li class="thumbnail stripe"&gt;Item 2&lt;/li&gt;
      &lt;li class="thumbnail stripe"&gt;Item 3&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 4&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 5&lt;/li&gt;
      &lt;li class="thumbnail"&gt;Item 6&lt;/li&gt;
      &lt;li class="thumbnail stripe"&gt;Item 7&lt;/li&gt;
      &lt;li class="thumbnail stripe"&gt;Item 8&lt;/li&gt;
      &lt;li class="thumbnail stripe"&gt;Item 9&lt;/li&gt;
  &lt;/ul&gt;
  </code></pre>
  
  <p>How can I go about this? I have something like this in mind, but I'm not sure what to put in the if statement.</p>
  
  <pre><code>$('#item-order li:visible').each(function (i) {
      if (...) $(this).addClass('stripe');
  });
  </code></pre>