sidenav.html

<ul class="usa-sidenav-list">
  {% for link in include.links %}
    {% assign _href = link.href %}
    {% assign _current = false %}
    {% if link.href == page.url or link.href == page.permalink %}
      {% assign _current = true %}
    {% endif %}
  <li>
    <a href="{% if link.external == true %}{{ link.href }}{% else %}{{ link.href | relative_url }}{% endif %}"
      class="
      {% if _current %}usa-current{% endif %}
      {% if link.class %}link.class{% endif %}
      {% if link.external %}link-external{% endif %}
      ">
      {% if link.iconbefore %}<span aria-hidden
          class="fa fa-{{ link.iconbefore }}"></span>
      {% endif %}
      {{ link.text }}
      {% if link.iconafter %}<span aria-hidden
          class="fa fa-{{ link.iconafter }}"></span>
      {% endif %}
    </a>

    {% if link.subnav %}
    <ul class="usa-sidenav-sub_list">
      {% include subnav.html links=link.subnav %}
    </ul>
    {% endif %}
  </li>
  {% endfor %}
</ul>