span></span><code>definitions are [ &#39;value&#39;, &#39;double&#39;, &#39;x&#39;, &#39;y&#39;, &#39;result&#39; ]
</code></pre></div>
</div>
<p>We think this approach to implementing the Visitor pattern is easier to understand and extend
than one that relies on callbacks,
but that could just be a reflection of our background and experience.
As with code style,
the most important thing is consistency:
if we implement Visitor using classes in one place,
we should implement it that way everywhere.</p>
<h2 id