plugins\pico_cascadenav.php

<?php

class Pico_Cascadenav
{

    public function get_pages(&$pages, &$current_page, &$prev_page, &$next_page)
    {
        $newPages = array('pages' => array());

        foreach($pages as $page) {
            $path = parse_url($page['url'], PHP_URL_PATH);
            $crumbs_main = explode("/", $path);
            $crumbs = array_filter($crumbs_main);
            if (count($crumbs) == 0) {
                $newPages = array_merge($newPages, $page);
                // if we have no subparts then skip ahead usually this is for /
                continue;
            }

            $key = array_pop($crumbs);
            $current = $item = array('pages' => array($key => $page + array('pages' => array())));
            
            while(count($crumbs) > 0) {
                $key = array_pop($crumbs);
                $current = array('pages' => array($key => $current));
                $item = array_merge($item, $current);
            }
            echo '<hr/>';

            $newPages = array_merge($newPages, $item);
        }
//        die();
        $pages = array($newPages);
//        echo '<pre>'; var_dump($pages);
//        die();
    }

}

themes\index.html

<ul class="nav">
                            {% include "menu.html" with {'pages':pages} only %}
  		</ul>

themes\menu.html

{% for page in pages %}
    <li>
        <a href="{{ page.url }}">{{ page.title }}</a>
        {% if page.pages %}
            <ul>
                {% include "menu.html" with {'pages': page.pages} %}
            </ul>
        {% endif %}
    </li>
{% endfor %}