How can one ensure that footnotes page-wrap only from verso (back, even-numbered) to recto (front, odd-numbered) pages?
With the following code, the line to which the second footnote is attached should be moved to page 4 so that the second footnote wraps from page 4 to page 5 (instead of from page 3 to page 5). The first footnote is okay (it spans from page 2 to page 3).
\documentclass{memoir}
\usepackage{lipsum}
\begin{document}
\lipsum Very long footnote.\footnote{\lipsum[1-5]}
\newpage
\lipsum[1-4] Very long footnote.\footnote{\lipsum[1-8]}
\end{document}
Some special considerations: (While extreme cases are unlikely to represent good style, (1) an implementer needs to be aware of them, and (2) some genres might give different priorities and not care about the oddity of such extreme examples. I have seen texts in the genre of legal commentary with many pages where footnotes fill most of the page.)
- An unresolvable case is when a footnote spans more than two pages. In that case it doesn't matter where it starts, though ensuring that it starts on a verso (even-numbered) page is a sensible choice too.
- If one has very long footnotes anchored to successive lines, one can simply fill the double page with the footnote and move each line containing such an anchor to the beginning of a new double page to fulfill the requirements.
- If one has very long footnotes anchored to words on the same line, there won't be much one can do about keeping the anchors on the same page as the very long footnote. In such a case, the line with footnotes 6, 7, and 8 will simply have do be on (say) page 2, with these footnotes starting on (say) pages 2, 6, and 8.
- An alternative and sensible way of ensuring the requirement is to move the footnote to the next verso (even-numbered page) while keeping the anchor on a recto page (this might make sense when there is a very long footnote on page 1 of a document).
These considerations will help with design decisions; there is more than one way to do it. Extreme cases won't occur much, but unless one works them out, it won't be clear how to implement an algorithm.