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Abayye: If one puts schach atop an exedra, it is kosher even though there are no posts because ‘pi tikrah yoreid v’soseim’. Rava: It is possul .

Summary

Rava to Abayye: So according to you, a sukkah can be made up of 2 parallel walls, as long as there is a board running across from one to the other? (It would seem so, because in the case of the exedra there are no walls at all!)

Abayye to Rava: In that case I would agree that it’s possul because it’s like an open alleyway. (The imaginary wall is erased by the fact that people are breaking through it all the time. But behind the portico the exedra has actual walls around it).

Challenge: It seems that Abayye and Rava are merely rehashing an old debate of Rav and Shmuel:

An exedra that is in an open valley (it is not surrounded by walls, but it does have two walls at a right angle):

Rav: One is permitted to carry within its bounds on Shabbos because of ‘pi tikrah’.

Shmuel: ‘Pi tikrah’ is not applied in this case.

Resolution: There is no doubt with regard to Shmuel’s opinion. Abayye and Rava’s argument depends on their understanding of Rav. Abayye holds that Rav always applies the principle of pi tikrah, so he applies it here as well. Rava explains that Rav applied the principle of ‘pi tikrah’ here in the valley because the walls were made especially for the exedra. But in the first case the walls are actually part of houses, and they are not there for the exedra at all.