Follow-up Comment #2, bug #61022 (project groff):
[comment #1 comment #1:]
> It certainly does seem like it was intended for a hook. The implementation
dates back to the dawn of repo time.
>
>
> ^351da0dc macros/tmac.s (James Clark 1991-06-02 04:20:34 -0500
1582) .de FP
> ^351da0dc macros/tmac.s (James Clark 1991-06-02 04:20:34 -0500
1583) .br
> ^351da0dc macros/tmac.s (James Clark 1991-06-02 04:20:34 -0500
1584) .if !d par*fp!\\n[FF] \{\
> 1294c8d22 tmac/s.tmac (G. Branden Robinson 2017-11-18 17:55:26 -0500
1585) . @error unknown footnote format '\\n[FF]'
> ^351da0dc macros/tmac.s (James Clark 1991-06-02 04:20:34 -0500
1586) . nr FF 0
> ^351da0dc macros/tmac.s (James Clark 1991-06-02 04:20:34 -0500
1587) .\}
> ^351da0dc macros/tmac.s (James Clark 1991-06-02 04:20:34 -0500
1588) .ie '\\$2'no' .par*fp!\\n[FF]-no "\\$1"
> ^351da0dc macros/tmac.s (James Clark 1991-06-02 04:20:34 -0500
1589) .el .par*fp!\\n[FF] "\\$1"
> ^351da0dc macros/tmac.s (James Clark 1991-06-02 04:20:34 -0500
1590) ..
>
>
> ...but we've gotten along for 30 years without exposing it in
documentation...do we really need it? Is it worth the clash with Plan 9?
(Although I have to say, using any amount of API on the ultra-legacy font
mounting position feature seems like a waste to me.)
>
> You certainly don't need .FP to do paragraphing within a footnote; the
normal ones work fine.
They do, until you need some sort of hook, to implement pdfhref links from the
footnote mark, to the footnote text, and back again!
> Do you agree? Should the comment be rewritten, maybe?
Well, the current support for footnotes, in s.tmac, is already inconsistent
with Mike Lesk's original implementation; he didn't have the "\**" string, and
there was no requirement for any similar construct, planted before ".FS", to
manage the footnote counter.
Some background may be helpful. I'm looking at pdfmark.ms, (which I've left
in a rather unsatisfactory state of incompletion for way too long). At the
bottom of page 2, (the introduction), are five footnotes; two of them are
linked, (from the mark to the footnote, but not back). Implementing even
those two forward links was a pain; "\**" itself is an impediment, because it
defies any attempt to wrap it as a pdfhref link, unless I abandon use of its
string invocation, in the manner it is documented.
To achieve a viable mechanism for implementing the forward footnote links, I
think we need to a) redefine "\**" as "\c", and b) provide a hook within "FS",
(which then *cannot* be separated from the mark location), to manage the
counter, and plant the mark as a pdfhref link. For the reverse link, we would
need a complementary hook within what is currently "FP", (whether that name is
retained, or some alternative is substituted).
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