Re: [Groff] what is `.if v' for?

[Dorai Sitaram]

Ralph Corderoy wrote
> 
> 
> Hi Dorai,
> 
> > The groff doc says `.if v' is always true both in groff and in
> > non-groff troff.  Does anyone know what the intent behind this
> > always-true conditional is?
> 
> Doesn't it always return false?
> 
>...
> 
> Don't know.  Just guessing.  But perhaps it's true in some other
> `troff', e.g. `vtroff'?

Yes, I should have said "always false".  

Is it guaranteed that groff/troff will not let the user
define a new alphabetic conditional (like
t,n,e,o,v) that is true?  (It looks like any
alpha that groff doesn't know about will be treated as
false.)

I ask because it seems like a fairly robust way to
introduce a conditional into a program other than groff
that parses groff text (i.e., the text can
distinguish whether it is being processed by groff or
the converter.)

Everything else (like using a num register or a string)
can be broken by the user.  One would have thought the
converter could posit a suitable device name for the
\*(.T string, but it turns out that \*(.T can be
changed by the user (why is this allowed?).