tbl was a real hack. It wasn't so much designed for the troff CAT output as it was
to drive nroff on a daisy wheel printer. Oddly, decades after I thought everbody
(including me as a last troff holdout) had abandoned it for more wisiwyg text formatters
someone sends me a manual. Tbl had a slight telltale glitch in that the vertical lines
on the left and right side of the table almost always protruded one pixel too high.
"Did you use tbl|troff to generate this?" I asked. Sure enough it was.