Implied question: We could have answered that the Mishnah teaches that the Chazakah is that he did not pay, i.e. within the time [it is due], or at the time, and he says "I paid you within the time", for there is no Migo, like I explained above (5b DH u'Va. He could not be brazen to say "I paid you today" for this is an obvious lie. One could not forget so quickly!)

However, after the time the Chazakah is that he paid, for we say Migo against a Chazakah.

Answer: He prefers to answer that there, he says "who says that Rabanan will obligate me to pay" [for the added height of the wall, since he only built a wall near it, and he did not yet use it], so it will not be difficult for Abaye and Rava [why he is not believed within the time].

Question (Ri): Here, why do we need the reason "anyone who says ['I did not borrow', it is as if he said 'I did not pay']"?

Surely, since he already admitted [that he owes], he is not believed to say that he did not borrow!

Granted, in Shevuos (41b) we need this reason. Reuven says "you owe me 100", and Shimon says "nothing ever happened (you never lent to me)", and witnesses come and say "we saw that he borrowed, and paid";

The witnesses are believed to say that he borrowed. They are not believed to say that he paid, for he contradicts them, for he said 'I did not borrow', and one who says 'I did not borrow', it is as if he said 'I did not pay'!

However, here we do not need this at all!

Answer (Ri): We need it to teach that Shimon, who said 'I did not borrow', cannot say 'I did not mean that I did not borrow at all. Rather, it is as if I did not borrow, for I paid';

[Our Gemara] teaches unlike this. It is as if he said explicitly "I did not pay."

Explanation: This "[he built close to] part" refers to both the length and the height.

Explanation (R. Tam): Karna is an indication that he will not lengthen the wall further. At the end of the wall that he built near (the wall in the middle), he makes a cornerstone pointing towards his neighbor's wall.

Lufta is an indication that he will not heighten the wall further, e.g. he made an indication at the top of the wall like they normally do, to make the top of the wall narrow above so the water will fall outside the wall and not delay there (on top of the wall).

Explanation (cont.): Rav Nachman agrees about Afriza. This is an indication that he will lengthen the wall further. At the end of the wall it is not even. Rather, one rock juts out and one is recessed. Surely, he will lengthen it further!

Akva'asa di'Keshurei means that he made a place to rest the ends of [ceiling] beams. It is proven that in the end he will heighten [his wall] to the amount of an Aliyah.

Explanation (R. Tam): He refers to our Mishnah. Until 30 days he has no Chazakah (to use it), and the Chazakah is that he did not give [money for it]. From now and onwards, the Chazakah is that he gave.

Support: This is why it was taught here, and not in Chezkas ha'Batim (Perek 3).

Observation: "Seven" is not precise, for he cannot take down [the Sukah] on Shemini Atzeres.

Explanation (Rashbam): The above Chazakos are not after three years, rather, immediately, like we say here.

Explanation (Ri): This is only when they are on two sides of Reshus ha'Rabim, and one does not sense when his friend goes up to his roof, to be careful from him (not to do matters that require privacy).

Source: [Shmuel] says below "however, between one roof and another, no [wall is needed]."

Note: One could say that Abaye argues with Shmuel and Rav Nachman. However, if so it is difficult why Abaye's opinion was brought first (he was a later generation). Also "Amar Rav Nachman" connotes a new matter. If he argued with Abaye, it should have said "Rav Nachman Amar."

Question: How was there a Havah Amina that he can say so? Just because he went first [and made half a wall], are his rights weakened, and the other can force him to make the entire wall?!

Answer: One might have thought that since had he not went first and made [half a wall], the other could have told him "take the expenditures (i.e. for half the wall) and make the entire wall [on your roof], or give to me the expenditures, and I will make the entire wall [on my roof]";

Both of us will profit. We will not need to build more than half.

Note: Also, when each builds more than half, only part of the roof is out of the other's view. If one builds the entire wall, both roofs are totally out of the other's view.

Also now [that he went first and made half a wall, one might have thought that], he can say so to him. [Abaye] teaches that he cannot say so, since he already made [half a wall], and the other did not tell him beforehand.

Question: Why must [the owner of] the lower [Chatzer] help [the owner of] the upper one? Once he reaches four Amos [above the ground of the lower one], the lower cannot see the upper Chatzer. [The owner of] the upper should build above this [alone]!

Answer #1 (R. Tam): The case is, the lower Chatzer is low only next to the upper one, and on the other side, it is the same elevation the upper Chatzer, so he can see there. (Therefore, he must help with the entire height of the wall in between.)

Answer #2 (Ri): The lower one damages the upper one (through Hezek Re'iyah) when the upper one stands, like we said above "when I stand [on my roof, people below in Reshus ha'Rabim] see me. Therefore, he must help [build the wall] up to four Amos [above the ground] of the upper Chatzer.