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1)

(a) If, in a *large* Sukah, one fills in part of a three Tefachim space in the S'chach, with metal rods - the Sukah is Kasher (seeing as there is neither the Shiur of space nor that of Pasul S'chach to invalidate it.

(b) If one did the same thing in a *small* Sukah - it would be Pasul, because it does not have the Shiur S'chach of a Kasher Sukah.

(c) If one filled in part of the space with canes, the Sukah is Kasher, because of the principle of 'Levud'. That is when it is *next to the wall*. Rav Acha and Ravina argue what the Din will be in the *middle* of the Sukah. According to one of them, the leniency in the former case only applies to the *side* of the Sukah, but not to the *middle* - because, in his opinion, 'Levud only applies next to a wall, but not to the middle of the roof (or the wall).

(d) Ravina proves that 'Levud' applies even in the middle of the roof (or the wall) from the Koreh of a Mavoy, where we validate two beams that protrude from the two parallel walls at the entrance of a Mavoy, which stop within three Tefachim of each other, because of 'Levud' - Rav Acha refutes this proof, on the grounds that this is restricted to the beams of a Mavoy, whose application is purely mi'de'Rabbanan.

(a) If a room has a Tefach by Tefach skylight, an object that is ...

1. ... directly underneath the skylight remains Tahor, if there is a piece of corpse in the room.
2. ... in the room, remains Tahor, too, if the piece of corpse is directly underneath the skylight.

(b) Ravina refutes the proof from here (that Levud does not apply in the *middle* of a wall or ceiling) - because Hilchos Tum'ah have their own specifications.

(c) Clearly, it was handed down from Sinai that with regard to Tum'ah, there is no Din Levud in the middle of a wall or ceiling - and one cannot learn extend what is learned from Halachah le'Moshe mi'Sinai to any other area of Halachah.

1. ... a Sukah with Pasul walls, provided there is at least seven by seven Tefachim of Kasher S'chach in the middle - that, according to his father, that applies only up to four Amos (where we say 'Dofen Akumah), but not more.
2. ... a type of small fish called Abruma - that, according to his father, it depended on the river: the Abruma of certain rivers was permitted, but of other rivers, it was forbidden.

4)

(a) Similarly, Abaye permitted a type of small fish called Tzachnasa from the River Bav. We initially thought that this was because the River Bav was a swift-flowing river - and non-Kasher fish, which had no spinal cord, could not survive in it.

(b) That is not the case, however - because non-Kasher fish were known to survive in similar swift-flowing rivers.

(c) Perhaps it was because the water was very salty, the Gemara then suggested, and non-Kasher fish, which had no scales, could not survive in it.

(d) The reason that we ultimately permit Tzachnasa from the River Bav - is not because we discovered them to be Kasher, but because the mud on its river-bed simply did not produce insects, which would render even Kosher fish, inedible.

6)

(a) If S'chach is placed on top of a courtyard that is separated from the adjoining passageway (which is more than four Amos wide) by means of posts that are placed at intervals of less than three Tefachim apart - the courtyard will serve as a Kasher Sukah, because, due to the Din of 'Levud', the posts are considered a proper wall.

(b) Abaye validates it even if there are no posts - because, in his opinion, we will apply the principle 'Pi Tikrah Yored ve'Sosem' to the inner vertical section of the ceiling that surrounds the courtyard.

(c) Rava does not apply 'Pi Tikrah Yored ve'Sosem' in this case (we shall see later why that is) See also Rabeinu Chananel.

(d) Abaye concedes that by a Sukah that has only two walls and S'chach consisting of planks of less than four Tefachim wide, 'Pi Tikrah Yored ve'Sosem' will not apply - due to the fact that, since it is open at both ends, the Rabbanan forbade it because it resembles an open Mavoy.

(a) Rav permits carrying on Shabbos even more than four Amos inside an Achsadra in a field that is not attached to a house - because he holds 'Pi Tikrah Yored ve'Sosem' (even by all four walls). Shmuel concedes that the principle of 'Pi Tikrah Yored ve'Sosem' by *three* walls, but not by *four* (See Tosfos DH 'Achsadra').

(b) We initially suggest that, in the Machlokes between Abaye and Rava - Abaye holds like Rav, and Rava, like Shmuel.

(c) We conclude however, that this is not the case. Even Abaye has to agree that only Rav will agree with him, but not Shmuel. We cannot possibly establish Shmuel by 'Achsasdra be'Bik'ah' exclusively, but by Sukah, he will agree that 'Pi Tikrah Yored ve'Sosem' - because if he holds 'Ein Pi Tikrah Yored ve'Sosem' by 'Achsasdra be'Bik'ah', where the walls *are* made for the inside of the Achsadra, then he will certainly hold that, by our case of a Sukah that is surrounded by an Achsadra, where they are *not*.

(d) Rava however, establishes that even Rav (who holds by Achsadra be'Bik'ah, 'Pi Tikrah Yored ve'Sosem' - because the walls *are* made for the inside of the Achsadra) will agree in the case of Sukah - where they are *not* that 'Ein Pi Tikrah Yored ve'Sosem'.