\section*{Declarations}

\subsection*{Data and code availability}
The sources of this paper and the exact-arithmetic program of
Appendix~\ref{app:check}, with its recorded output, are available at
\url{https://github.com/creelie/eCon}\releaseline.

\subsection*{Funding}
No funding was received for conducting this study.

\subsection*{Declaration of competing interest}
The author has no competing interests to declare that are relevant to the
content of this article.

\subsection*{Declaration of generative AI and AI-assisted technologies in the manuscript preparation process}
During the preparation of this work the author used Claude (Anthropic) to
write and run the program of Appendix~\ref{app:check} and to draft text.
After using this tool, the author reviewed and edited the content as needed and
takes full responsibility for the content of the publication.