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<ul>
  {% for nav in site.data.navigation %}
    {% if nav.children != null %}
      <li><a href="{{ nav.href }}">{{ nav.title }}</a>
        <ul class="child">
        {% for child in nav.children %}
          <li><a href="{{ child.childhref }}">{{ child.childtitle }}</a></li>
        {% endfor %}
        </ul>
        {% else %}
      <li><a href="{{ nav.href }}">{{ nav.title }}</a>{% endif %}</li>
  {% endfor %}
</ul>

# example /_data/navigation.yml
- title: "About"
  href: "/about/"
  children:
    - childtitle: "Biography"
      childhref: "/about/bio/"
    - childtitle: "Resume"
      childhref: "/about/resume/"

- title: "Portfolio"
  href: "/portfolio/"
  children:
    - childtitle: "Design"
      childhref: "/portfolio/design/"
    - childtitle: "Illustration"
      childhref: "/portfolio/illustration/"
    - childtitle: "Development"
      childhref: "/portfolio/development/"