{% macro toc(headers) %}
  <ol start="0">
  {%- for header in headers -%}
    <li><a href="{{ header.permalink }}">{{ header.title }}</a>
      {%- if header.children | length > 0 -%}
      {{ self::toc(headers=header.children) }}
      {%- endif -%}
    </li>
  {%- endfor -%}
  </ol>
{% endmacro toc %}