nroff question on italics/underscore

[Dick Dunn]

I recently discovered that nroff underscores text (as when it's doing its
"italic" font) with '_' followed by \b followed by the character.  The
thing that seems odd is that it uses, literally, '_'.  It DOESN'T use the
character given for '_' in the mapping table (in /usr/lib/term/tabxyzzy),
nor does it use \(ru (rule) or \(ul (underrule)--the char constant is
hardwired into the code.

(This is on a 4.2 system, to the extent that might matter.)

Any idea why it's done this way?  In particular, any idea whether it was
intentional or just lazy?

-- 
Dick Dunn	{hao,ucbvax,allegra}!nbires!rcd		(303)444-5710 x3086
   ...Simpler is better.