nroff - help wanted

[Peter King]

The problem of having nroff emit a form feed was solved here
using an undocumented request .cf which copies its file argument
to the nroff output stream, without performing any processing on it.

The page header macro is then modified to include the line
.cf /usr/lib/tmac/ff
and the file /usr/lib/tmac/ff is set up containing a form-feed
character.
As far as I can make out, the restrictions on the characters on
input that nroff will recognise make it very difficult to get
a control character in your output in any other way.

Although .cf is not documented it has been present in all the
nroff's I have tried. Does any body not have it?