lass="identifier">last</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
        <code class="computeroutput"><span class="identifier">position_cache_tag</span></code> is a special
        tag we will use to get a reference to the actual <code class="computeroutput"><span class="identifier">position_cache</span></code>,
        client data that we will inject at very start, when we call parse. More on
        that later.
      </p>
<p>
      