[bug #61683] [me] either document $r and $R registers or rename them

[G. Branden Robinson]

Follow-up Comment #3, bug #61683 (project groff):

[comment #1 comment #1:]
> [comment #0 original submission:]
> > Or maybe someone will be able to locate a use case.
> 
> If that sideways glance was in my direction, I don't have one.

Not this time.  :)

> I had never spotted this register in the source, and can't think how to use
it now that I know about it.  Maybe it's some experimental dead end Eric was
pursuing in the '70s that's since become ossified.
> 
> On the other hand, unless you plan to repurpose the registers for something
else, leaving them as-is ensures they'll continue to work for anyone who _is_
using them but not following groff discussion channels.  I've been known to
exploit undocumented -me internals on occasion to fiddle with something for
which there's no approved mechanism, under the assumption that -me internals
in the 21st century are largely unchanging.  Worth a code comment to note the
anomalous name, but beyond that perhaps doesn't warrant a change.

I've figured out that they exist for BSD me(7) document support; I infer that
Clark retained these register names deliberately.

So I will leave them.

> (If your itchy refactoring finger needs an outlet,

Maybe "inlet" is a better analogy, with as much poking around as I do...

> I'd look at the ?k register, which is set in multiple places but appears to
be never used -- at least under that name, though because the language lets
you build register names on the fly, a simple code examination is insufficient
to determine this.)

That's true, though from what I've seen, me(7) is pretty light on dynamic
symbol name construction.  It doesn't try to maintain arrays of things, for
example.

I had a quick look around at `?k`.  I _think_ it's a Boolean that answers the
question "am I in a keep?".  I agree that nothing seems to ever test or
interpolate it.

An example document that exercised the following types of keep or annotation,
coupled with an e.tmac patched to remove all settings of the variable, and
rendered with `groff -ww`, would, I think, test my hypothesis effectively.

(q, )q: long quotation
(l, )l: list
(z, )z: floating keep
(b, )b: block

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