Generation process for cpp.1

[Joe Buck]

> > Um, no, I meant \fP, which switches to the previous font.  This only
> > works if there's been exactly one font change, if you do a second one
> > you're hosed.
> 
> Oh... I didn't know you could do that.  Do you know how widespread support
> for that is?  It's not in the troff paper ...

\fP has been in every nroff/troff implemenation I have ever seen, and
I used to use it regularly in some roff stuff I wrote back in 1984 or
before.  (OK, I'm old).  Also, it's used heavily to implement the
various macro packages.

It is not an extension.  But there is no stack, it remembers only one
previous font.