`\Include` `parent` argument

This option is analogous to \H parent argument, but for includes.

For example, consider you have:

= Animal

== Dog

== Cat

== Bat

and now you want to split Cat to cat.bigb.

If you wrote:

= Animal

== Dog

\Include[cat]

== Bat

Cat would be a child of Dog, since that is the previous header, which is not what we want.

Instead, we want to write:

= Animal

== Dog

\Include[cat]{parent=animal}

== Bat

and now Cat will be a child of Animal as desired.

 Ancestors