Question #1: It is evident in 'Yesh Nochlin' (Bava Basra 115:) that from this Pasuk we learn the principle 'B'nei Banim ke'Banim', so how can it serve as a proof that he had relations with his mother?

Question #2: Moreover, nowhere in the Torah do we find that someone called his wife's son 'his son', yet here the Gemara says that the Torah calls him his son, because he was born to his wife?

Answer: It calls him 'his son', because he was his son's son, as we learned in 'Yesh Nochlin'.

Answer (cont.): And the Gemara here infers that he had relations with his mother, since the Torah omits 'Ayah', who was also the son of Tziv'on, and it only mentions 'Anah' ...

Answer (concl.): A clear indication that it is because he (Anah) was also the son of the wife of Se'ir that the Torah inserts him here together with the other sons of Se'ir.

Clarification: Rashi explains that it refers to stopping eating whilst it is still day, to forbid work and the other Chumros mentioned there (inTa'anis).

Question: Since when is it prohibited to do work on Tish'ah be'Av, like a Ta'anis Tzibur?

Answer: What the Gemara means is that there is no Ta'anis Tzibur in Bavel, even in a town where it is customary not to do work ... .

Implied Question: Why does it not mention Melachah, which is permitted on Tish'ah be'Av in a town where that is the custom.

Answer: The Ri explains that this is because the Seifa will conclude that Melachah is permitted, when it discusses the differences between Tish'ah be'Av and a Ta'anis Tzibur.

Implied Question: And the reason that it does list Tefilas Ne'ilah, which is not said on Tish'ah be'Av, as we will learn later ...

Answer: Is because the Beraisa is confined to cases of Isur and Heter.

Question: The Ri asks why, does it mention Safek day with regard to Yom Kipur, seeing as even when it is Vaday day it is forbidden, bearing in mind that Tosefes Yom ha'Kipurim is d'Oraysa?

Answer: And he answers the Tana mentions Safek, to teach us that even Safek day, Safek night is permitted.

Refuted Explanation: One cannot explain this with regard to the B'nei Bavel, ho do not know when Rosh Chodesh was fixed in Eretz Yisrael, and who therefore need to observe two days Yom Kipur ...

Refutation: Because the Gemara at the end of the first Perek of Rosh ha'Shanah (21.) specifically writes that they only need to observe one day.

Explanation: Consequently, 'S'feiko Asur' - if one has a doubt, even in Eretz Yisrael when Rosh Chodesh was fixed (for example someone who is travelling in a desert), he must observe two days, like all S'feikos d'Oraysa, where we go le'Chumra.

Question: The Rashba queries the implication that regarding 'Kevi'a de'Yarcha' they are different, but regarding all other issues, they are equal ...

Question (cont.): In that Tosfos Yom Kipur is forbidden, whereas Tosfos Tish'ah be'Av is permitted?

Answer: The Ri answers that the Tana is only speaking about matters that concern the fast itself.

Implied Question: ven though Rebbi Yochanan is speaking specifically about somebody who is not sure whether he Davened or not. But someboidy who already Davened, is forbidden to Daven again, as is evident in 'Mi she'Meiso' (B'rachos 21.) ...

Proof: Where it says 'Safek Hispalel, Safek Lo Hispalel, Al Yispalel. Ve'Rebbi Yochanan Amar "u'Levai she'YispalelKol ha'Yom'.

Proof (cont.): And it adds there that someone who has already Davened and finds the Tzibur Davenning, is permitted to Daven with them, only if he is able to insert something new in his Tefilah, but not otherwise?

Answer: Nevertheless, the Gemara's Kashya is justified. This is because, seeing as Rebbi Yochanan obligates one to Daven in a case of Safek, Tish'ah be'Av, which is similar to a Ta'anis Tzibur, in many ways, ought to have a Tefilas Ne'ilah, like it.

Answer (concl.): And the Gemara answers 'There it is an obligation, whereas here, it is voluntary - not absolutely voluntary, but it is not obligatory, just a Mitzvah.