{% comment %}
The nav-tree variable has been defined in the include that called this file.
To include child items in that tree, this file includes itself, 
defining nav-tree as the children of the nav-tree it received originally.
Turtles all the way down, at least as deep as the nav tree in meta.yml goes.
Credit to Christian Specht for the inspiration:
https://christianspecht.de/2014/06/18/building-a-pseudo-dynamic-tree-menu-with-jekyll/
{% endcomment %}

<ol>
{% for item in include.nav-tree | sort: "order" %}
    <li>
        <a href="{{ base.url}}{{ item.file }}.html{% if item.id != nil %}#{{ item.id }}{% endif %}"
           class="{% if page.url contains item.file %}active{% endif %}">
            {{ item.label }}
        </a>
    </li>
    {% if item.children != nil %}
            {% include nav-list nav-tree=item.children %}
    {% endif %}
{% endfor %}
</ol>