AST with its position in the input stream with
        our <code class="computeroutput"><span class="identifier">on_success</span></code> handler:
      </p>
<pre class="programlisting"><span class="comment">// tag used to get the position cache from the context</span>
<span class="keyword">struct</span> <span class="identifier">position_cache_tag</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">annotate_position</span>
<span class="special">{</span>
    <sp