formation that we can use to tell the AST's position
        in the input stream anytime.
      </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">client</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">ast</span>
<span class="special">{</span>
    <span class="keyword">struct</span> <span class="identifier">person</span> <span class="special">:</span> <span class="identifier">x3</span><span class="special">::