Lost a number of hours, too many, on a mysterious LaTeX problem. It turns out that fitch.sty (useful for setting Fitch-style proofs) loads a package mdwtab.sty which isn’t fully compatible with the standard tabular environment (useful for setting truth-tables). So when I added fitch.sty to the preamble to the book source, a few truth-tables mysteriously lost horizontal lines. Took me longer than it should have done to find the culprit. Warning, and a solution, now added to the page on natural deduction packages in LaTeX for Logicians.

Right: back to thinking a bit about how I want to handle empty domains later …