{%- macro render_nav_items(items, depth=0) %}
<ul class="md-nav {% if depth > 0 %}md-nav--nested{% endif %}">
  {%- for nav_item in items %}
  <li class="md-nav__item">
    {%- if nav_item.children %}
    <span class="md-nav__link">{{ nav_item.title }}</span>
    {{ render_nav_items(nav_item.children, depth + 1) }}
    {%- else %}
    <a
      href="{{ nav_item.url | url }}"
      class="md-nav__link {% if nav_item.active %}active{% endif %}"
    >{{ nav_item.title }}</a>
    {%- endif %}
  </li>
  {%- endfor %}
</ul>
{%- endmacro %}
{{ render_nav_items(nav) }}