h3>
<p>Derive a class from <code>Walker</code> that reports how deep each top-level expression in the source code is.
For example,
the depth of <code>1 + 2 * 3</code> is 2,
while the depth of <code>max(1 + 2 + 3)</code> is 3
(one level for the function call, one for the first addition, and one for the nested addition).</p>
<h3 class="exercise">Downward and upward</h3>
<p>Modify <code>Walker</code> so that users can specify
one action to take at a node on the way down the tree
and a separate action to take o