Eruvin 12B

הכא במאי עסקינן ⁠— דאית ליה גידודי.

The Talmud answers: With what are we dealing here in this baraita? It is a case where the wall has not been fully breached, but rather remnants of the wall remain on each side (Rabbeinu Ḥananel; Rif).

אמר רב יהודה: מבוי שלא נשתתפו בו, הכשירו בלחי, הזורק לתוכו ⁠— חייב, הכשירו בקורה ⁠— הזורק לתוכו פטור.

Rav Yehuda said: If several courtyards open onto a common alleyway, the residents of the houses in the courtyards are prohibited to carry in the alleyway, unless the alleyway is rendered fit for one to carry within it by placing a side post or a cross beam at its entrance, and by the inhabitants of each courtyard placing food in a common area for the duration of Shabbat, symbolically converting the entire alleyway into a single household. It is prohibited to carry in an alleyway that the residents did not merge. Nevertheless, if the alleyway was rendered fit by means of a side post placed at its entrance, one who throws an object into it from the public domain is liable; the side post functions as a partition, and the alleyway is deemed a full-fledged private domain. If, however, the alleyway was rendered fit by means of a cross beam, one who throws an object into it from the public domain is exempt; the cross beam functions only as a conspicuous marker. It is not considered a partition that renders the alleyway a private domain.

מתקיף לה רב ששת: טעמא דלא נשתתפו בו, הא נשתתפו בו אפילו הכשירו בקורה נמי חייב?! וכי ככר זו עשה אותו רשות היחיד או רשות הרבים?

Rav Sheshet strongly objects to this due to the following: The reason that one is exempt in the latter case is due to the fact the residents of the alleyway did not merge. By inference, if they did in fact merge, one would be liable even if the alleyway was rendered fit by way of a cross beam. This, however, is difficult. One can ask: Does this loaf, through which the residents joined together to form a single household, render the alleyway a private domain or a public domain?