Moving things around "opposite the gap" in an enclosure on Shabbat
On Shabbat there are mitigating structures whose presence can enclose an area physically, constituting a necessary ingredient in allowing movement of objects within that area.
There are several shapes and sizes of gaps in the enclosure that do not invalidate it. (See some details here.) In discussing the parameters of the permissible gaps, many writers on the topic state that moving things around is allowed "[even] opposite the gaps", and not only "opposite the solid part".
Ra'av on Eruvin 1:8
פרוץ כעומד, הוי מחיצה ואף כנגד הפרוץ מותר
Mishna B'rura 362:48
ואפילו כנגד הפרוץ מותר לטלטל דיש ע"ז דין מחיצה שלמה
These word choices are presumably based on the same phrasing that appears in the discussion of barriers that allow the planting of crops whose proximity would otherwise be forbidden. The two bodies of law are explicitly used to inform one another, and some of the qualifications for barriers are carried over to enclosures.
E.g. Y'rushalmi Eruvin 1:8
,לענין כלאים כל הפחות מג' כסתום. מג' ועד ארבעה אם העומד רבה על הפרוץ מותר ואם הפרוץ רבה על העומד אסור. מארבע ועד עשר אם העומד רבה על הפרוץ מותר, אם הפרוץ רבה על העומד כנגד העומד מותר, כנגד הפרוץ אסור. יותר מעשר אע"פ שהעומד רבה על הפרוץ כנגד העומד מותר. כנגד הפרוץ אסור
It is clear what "opposite the gap" means when dealing with where one may not put seeds in the ground because the whole purpose of the barrier is to separate/delineate between two nearby groups of plants.
When the phrase is applied to moving things around the campsite on shabas what does "opposite the gap" mean? Some hypotheses that come to mind are
just inside the gap and up close to it, exactly like in the plants case (which would be strange as conceptually none of these laws have to do with things happening up close to the enclosing structure),
anywhere in the airspace bounded by the ends of the gap and projected inward perpendicular to the wall that the gap is in (which just seems odd),
in the airspace between two gaps that are across from one another (along a line parallel to the closest edge between them? The existence of such a space is significant in nearby discussions, so maybe it's relevant.).
None of these is obvious and they are all a bit more of a stretch than I would expect for nobody to say anything, which is what I've found.
The question has no practical application, as it only comes up as a consideration to be excluded (a hava amina?). It is a methodological one.
commentaries maseches-eruvin
DonielF
This hava amina doesn't have to actually make sense. It could just be there to make sure people aren't unnecessarily machmir – Heshy Nov 13 '18 at 19:08
I considered that, and would accept it if there were no better explanation. But it doesn't sit well that it would come up in several places without its authors really meaning it. – WAF Nov 13 '18 at 19:16
Which daf is the relevant Bavli? Or is there no Bavli on this? – user18155 Nov 14 '18 at 18:29
It's referring to the opening in the enclosure; that's it's permitted to carry there "against" it even though there is no enclosure(like a string between two posts currently used in a community eruv). – chacham Nisan Nov 14 '18 at 18:38
@DannyF 16, but the parallel between topics is not quite as obvious as in the Y'rushalmi. – WAF Nov 14 '18 at 20:14
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