AT&T troff requests

[Humm]

After stumbling upon the .li request in v6 man pages, I took a little 
look at what requests were removed and added between v7 and Plan 9 4E 
and at what they did.

.li and .fz went away; .fm, .gd, .ha, and .pt were introduced.  As far 
as I can tell, none of them are mentioned in the Troff User’s Manual 
(CSTR 54, Plan 9, and Heirloom versions), in Mandoc docs, or in Groff 
docs.  I suggest mentioning these somewhere in the docs, even if not 
implemented.

.fm, .gd, .ha, and .pt are part of Plan 9 troff and thus usable via 
9front and via plan9port.  .li and .fz can be used in a v7 VM.  The 
FIXES file mentioned later is part of the Plan 9 and plan9port source 
trees ((/sys/)src/cmd/troff/FIXES).

Hereafter, I describe what I found about the requests.  (Take it with 
a grain of salt, as I don’t know my way around troff sources well.)

I’m not sure what .fz did.

`.li N` read N (default 1) lines literally.

        .li
        .ab

would have the same effect as

        \&.ab

`.ha N` sets the hyphenation algorithm.  The FIXES file, entry from 
January 1991:

> added tex hyphenation, using standard tex data files, but not the
> elaborate compressed trie, which is a lot of trouble to save maybe
> 40k bytes.  this appears to run at exactly the same speed as before.
>
> […]
>
> the command .ha sets the algorithm.  .ha 1 => tex, with troff rules
> if tex doesn't hyphenate;  .ha 0 gives troff rules, and .ha resets
> to the default, which is tex.  the hyphenation algorithm is part of
> the environment, a nod to a future in which i handle more than one
> language.
>
> […]

`.fm filename string` writes string to a file.

`.pt N` sets the trace mode.  The FIXES file, entry from 1992-07-12:

> added .pt request to trace macros and requests (from jaap).
> .pt N Print trace of macros (N=1), requests (N=2) or both (N=3)

and from 1992-10-26:

> Numeric arguments error reporting:
>
> Controlled by .pt, illegal numbers are now reported (default
> trace mode is 1).  This is also true for the escapes:
> \h'..', \v'..' \H'..', \S'..', \N'..', \D'..', \l'.., \L'..
> and \x'..'.
>
> […]
>
> Enhancement to .pt:
>
> The .pt now pops the previous values when no argument is
> specified. Turned out to be handy when chasing for problems.
> Just ``bracked'' the code with .pt 7 and .pt and you get
> a trace of only that block. The meaning of the arguments
> is now:
>         01      trace numeric arguments (default on)
>         02      trace requests
>         04      trace macros
>
> […]

`.gd xx` sets number registers dn and dl to the size of diversion xx.

--
Humm