All you really need to know is that this is cursed.

followups.dtl

{% block followups %}
  {% set dir = page.components | slice(end=-1) | join(sep="/") %}
  {% set parent = page.ancestors | last %}
  {% set section = get_section(path=parent) %}
  {% set followups = [] %}
  {% set a = page.extra.followup_groups | default(value=[]) | sort %}
  {% set a_sz = a | length %}

  {# Add explicitly listed pages to follow-up list. #}
  {% for stem in page.extra.followups | default(value=[]) %}
    {% set filename = dir ~ "/" ~ stem ~ ".md" %}
    {% set other = get_page(path=filename) %}
    {# We need to use `set_global` because of scoping rules. #}
    {% set_global followups = followups | concat(with=other) %}
  {% endfor %}

  {% if a_sz %}
    {# Search through other faq pages for those in our follow-up groups. #}
    {# Yes, this is O(n^2), but that's fine with small coefficients! #}
    {% for other in section.pages %}
      {% if other != page and other not in followups %}
        {% set b = other.extra.groups | default(value=[]) | sort %}
        {% set b_sz = b | length %}
        {% set_global i = 0 %}{% set_global j = 0 %}
        {# No `while` loop support, so do some stupid shit. #}
        {% for _ in range(end=999) %}
          {# Break out of the loop at the end of either array. #}
          {% if i >= a_sz or j >= b_sz %}
            {% break %}
          {% elif a[i] == b[j] %}
            {% set_global followups = followups | concat(with=other) %}
            {% break %}
          {% else %}
            {# Advance one of the lists, depending on comparison. #}
            {# This is just `if a[i] < b[j]` with extra steps. #}
            {% set compare = [a[i], b[j]] | sort | first %}
            {% if compare == a[i] %}
              {% set_global i = i + 1 %}
            {% else %}
              {% set_global j = j + 1 %}
            {% endif %}
          {% endif %}{# state handling #}
        {% endfor %}{# range #}
      {% endif %}{# eligible? #}
    {% endfor %}{# other #}
  {% endif %}{# a_sz #}

  {# ACTUAL CONTENT GOES HERE! #}
  {% if followups | length %}
    <h3>Follow-up questions:</h3>
    <ul>
    {% for other in followups %}
      <li><a href="{{ other.permalink | safe }}">{{ other.title | safe }}</a></li>
    {% endfor %}
    </ul>
  {% endif %}
{% endblock followups %}