If any external URLs are linked in a chapter then they are displayed and linked below, organized by chapter name.
The shell script which is used to generate this list is named:
generate-url-index.sh : This bash program looks for external reference links.
: The link must occur at the start of a line.
: The link conforms to Markdown norms for reference links; that is:
[a-phrase-without-whitespace]: https://some.link.somewhere/something
Then the script outputs each link by full name, including the digital web link to itself. It might be useful in print-only media.