Add the ability to arbitrarily inject one of the related topics by including a comment like:

<!-- Inject:44 -->

Where 44 is a TopicID. Skynet injects an xref to TopicID 44.
<xref linkend="[sectionID]"/>

<!-- InjectSequence:44,45,67 -->

Skynet injects:

<orderedlist>
[for each TopicID]
  <listitem><para><xref linkend="[sectionID]"/></para></listitem>
[end:for]
</orderedlist> 

<!-- InjectList:44,45,67 -->

Skynet injects:

<itemizedlist>
[for each TopicID]
  <listitem><para><xref linkend="[sectionID]"/></para></listitem>
[end:for]
</itemizedlist>