By
{% assign last_pos=include.authors.size | minus: 1 %}
{% assign and_pos=include.authors.size | minus: 2 %}
{% for i in (0..last_pos) %}
  {% assign author_id=include.authors[i] %}
  {% assign author=site.authors[author_id] %}<!--
  -->{% if author %}<!--
       --><a href="{% if author.url %}{{ author.url }}{% else %}mailto:{{ author.email }}{% endif %}">{{ author.name }}</a><!--
  -->{% else %}<!--
       -->{{ author_id }}<!--
  -->{% endif %}<!--
  -->{% if i == and_pos %}<!--
       --> and <!--
  -->{% elsif i < and_pos %}<!--
       -->, <!--
  -->{% endif %}
{% endfor %}
on <span class="text-muted">{{ include.date | date_to_long_string }}</span>