rray.
    If our node handling functions don&rsquo;t require any extra data
    from one call to the next
    we can leave this out;
    if we want to accumulate information across calls,
    this argument acts as the Visitor&rsquo;s memory.</p>
</li>
</ol>
<p class="continue">Any general-purpose implementation of the Visitor pattern
is going to need these four things,
but as we will see below,
we can implement them in different ways.</p>
</div>
<figure id="style-checker-walk-tree" class="figure-here">
  <i