Inference: Rabanan hold that a plank Mashehu (of any size) suffices even for a square Chatzer, for they argue about a square Chatzer, like I will explain below (12a DH v'Rav).

Explanation #1: "Two Tefachim" is not precise, for we say below (12a) that in two directions, Mashehu on each side [suffices];

Implied suggestion: Perhaps "Mashehu" is not precise, like we say below (80a) regarding Shituf Mavo'os "one must raise it Kolshehu. How much is Kolshehu? It is a Tefach."

Rejection: If so, it should have explained, like there.

Explanation #2: For a proper plank, two Mashehu's (one on each side) suffice. However, the wall itself is considered a plank only if it is a Tefach wide, for there is not as much Heker through a wall as through a proper plank.

Explanation #1: If from the inner end of the plank until the middle wall is more than 10 [Amos], this Mavuy needs Tzuras ha'Pesach, for it is like a bent Mavuy.

Even though we discuss according to Shmuel, and it said (6a) "if it is more than 10, would Shmuel say that it is like a closed [Mavuy]?!", I explained above (6a DH Rav) that there it discusses [the width of] the opening where it is bent.

Explanation #2 (Ritzva): Here, even more than 10 is not called Mefulash, since both [ends] open to one Reshus ha'Rabim, and one who stands in this opening does not go turn around to go around the plank to the other opening;

However, in a bent Mavuy, there are in the two ends on each side [a different] Reshus ha'Rabim, and it is common to go through the Mavuy from one side to the other. Therefore, it is Mefulash.

Implied question: Why did they obligate doors for a bent Mavuy (6b, which Rashi explained that it was) shaped like the letter 'Ches'?

Answer #1: Its two openings were not open to one Reshus ha'Rabim (rather, to two Reshuyos ha'Rabim).

Answer #2: [The Ritzva] will explain [unlike Rashi] that it was Mefulash from three directions, and in the middle is bent, like the Ri explained (6a DH Rav).

Explanation: He needs another Lechi, for this plank does not help, since it is distanced [even more than] three Tefachim from the wall. It says below (14b) that then it is not a Lechi;

Therefore, he asks from R. Ami and R. Asi (5a. If a side wall of a Mavoy near the opening was broken, and four Tefachim do not remain, if the breach is at least three Tefachim, they forbid, lest the breach become the primary opening, and the Lechi or Korah was not made for it. Also here,) the Tikun (Lechi or Korah of this Mavuy) should be Batel [due to this concern]!

Explanation #1 (Rashi): [He asks] why did he obligate a plank three Amos together?

Inference: The Makshan comes to be lenient.

Rebuttal: It connotes unlike this, for below we answer "Rabanan did not burden him." This connotes that the Makshan comes to be stringent on him! All the questions connote like this.

Explanation #2: He asks that he should make a plank one and a half Amos [and another such plank two Amos away], for this is better, for when there are Gifufei on both sides, it looks more like a Pesach.

Explanation: This is when the Tum'ah and the Kli are under the hide, like we say in Sukah (18a) if an skylight is in a house, and it has an open [square] Tefach, if Tum'ah is in the house, what is in line with the skylight is Tahor. If [Tum'ah is] is in line with the skylight, the entire house is Tahor.

Even so, the air joins, just like air of a Sukah (a space without any Sechach), which joins (to the Shi'ur of a Sukah), but one may not sleep under it.

Implied question: Why don't we consider the air to be closed, due to Lavud?

Answer: There is a tradition regarding Tum'ah, that we do not say Lavud.

Support: This is why below it says that if there are two fingers here [and two fingers here...] If you would say Lavud, it suffices Mashehu on each side!

Explanation #1: Regarding the hide of a toilet seat, which is a Kli, we do not say "the air on this side and the Stam air (outside it) come and are Mevatel it." We say so only regarding fixed Mechitzos, like above, and below (16b), regarding "we surround with ropes."

Explanation #2 (Ritzva): We say that Stam air comes and is Mevatel only regarding air that I consider like sealed, like the case above in which air of 10 is considered like sealed, and surrounding with ropes. The air above 10 [Tefachim] is considered sealed;

However, air that is not from the wall is not Mevatel.

Support: It connotes like this above (5a). R. Ami and R. Asi said that if there is a plank four [Tefachim] wide, it permits a breach up to 10 [Amos]. We do not say that the air on this side and the Stam air (outside it) come and are Mevatel it.

Disclaimer: If we would distinguish between the initial [Heter] of a Mavuy and [fixing] a Mavuy that already was [permitted], there is no support.