an class="w">  </span><span class="c1">// Act on node and then on children.</span>
<span class="w">  </span><span class="nx">_walk</span><span class="w"> </span><span class="p">(</span><span class="nx">node</span><span class="p">,</span><span class="w"> </span><span class="nx">accumulator</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w">    </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="nx">node</span><span class="w"