title: Hide reveal.js fragments after their appearance
body: <p>Say I have a simple reveal.js slide like this:</p>
  
  <pre><code>&lt;section&gt;
    &lt;h2 class="fragment" data-fragment-index="1"&gt;first&lt;/h2&gt;
    &lt;h2 class="fragment" data-fragment-index="2"&gt;second&lt;/h2&gt;
    &lt;h2 class="fragment" data-fragment-index="1"&gt;first&lt;/h2&gt;
  &lt;/section&gt;
  </code></pre>
  
  <p>I want the two "first" to be shown <strong>only</strong> in fragment 1 and then go back hidden in fragment 2, when "second" appears. How should I do that?  </p>