Logic

LaTeX: Numberless lines in fitch.sty

To write natural deduction proofs in LaTeX, I use a package called fitch.sty. The package was written by Johan W. Klüwer and offers a nice clean way to typeset Fitch-style proofs. He provides a nice example:

Lovely. However, in some of my proofs, I wanted to have lines without numbers because they featured information that was not strictly part of the proof. For instance, like others, I commonly add a line that indicates the formula we’re out to prove after the list of premises. This is especially useful in teaching proofs. That line, I don’t want numbered — instead I want the counter to skip that line and continue after it, like so:

I had to dig around in the fitch.sty file itself to figure out how to do this, since there’s not really any documentation outside of it. I figured I’d share what I did for anyone facing the same issue.

Here’s what you do. Instead of beginning a line with “\fa” or something like that, add a line like this:

\ftag{~}{\vline\hspace{\fitchindent} CONTENT } \\

Where CONTENT is replaced by whatever you want to have on that line. The exact code for the my ‘∴ B’ line, for example, is:

\ftag{~}{\vline\hspace{\fitchindent} \fbox{$\therefore~ B$}} \\

And that’s all there is to it. I hope this helps someone looking to do the same thing as I was.

Happy typesetting!

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