Chapters can be stand-alone documents or parts of a book
In a book you frequently refer to other chapters
When a chapter is a stand-alone document, it must refer to
another stand-alone chapter document
This requires generalized cross-referencing
LaTeX has a solution: xr
DocOnce has a solution also for HTML, Sphinx, Markdown, ...
ref[internal][latex cite][external]
...as shown in ref[Section ref{sec:eqs}][ in cite{math_eqs_2020}][
the document "Mathematical Equations":
"http://some.net/doc/matheqs.html" cite{math_eqs_2020}].
