<a class="x-ref" href="../page-templates/">Chapter 9</a>.
If we provide a function to act on nodes of type <code>Identifier</code>,
<code>acorn-walk</code> will call that function each time it finds an identifier.
We can use other options to say that we want to record the locations of nodes (i.e., their line numbers)
and to collect comments in an array called <code>onComment</code>.
Our function can do whatever we want;
for demonstration purposes we will add nodes to an array called <code>state</code>
and r