bug using doublebox in tbl with ditroff

[Jeffrey Copeland]

Your problem with tbl and ditroff sounds more like the line drawing 
characters aren't tuned correctly.  The four characters, em, rn, ul,
and ru should all meet at their ends, so horizontal lines can be drawn.
The vertical character br should meet the rn on the top and the ul on
the bottom.  ul should run under the baseline; ru at the baseline.
If the characters are tuned properly, the "smallest possible constructed
box", \(br\z\(rn\(ul\(br (see Troff User's Man, sect 12.2) will have 
corners that match.