1)

(a)We learn from the Pasuk "me'Im Mizbechi Tikachenu la'Mus" - that Retzichah overrides Avodah.

(b)We now learn from a 'Kal va'Chomer' that Retzichah (Misas Beis-Din) ought to override Shabbos - because if Retzichah overrides Avodah, which overrides Shabbos, then it should certainly override Shabbos.

(c)Assuming that Kevuras Mes Mitzvah does not override Shabbos, we refute the 'Kal va'Chomer' that Retzichah ought to, because it overrides Avodah - by citing Kevuras Mes Mitzvah, which overrides Avodah, but not Shabbos.

(d)We counter this (and re-instate the 'Kal va'Chomer') - by suggesting that on the contrary, by virtue of the same Kal-va'Chomer', Kevuras Mes Mitzvah ought to override Shabbos, since it overrides Avodah which overrides Shabbos!

2)

(a)We initially thought that the Beraisa's original suggestion that Retzichah overrides Shabbos is based on the principle 'Aseh Docheh Lo Sa'aseh'. The Tana reconsiders this however - on the grounds that we only say that by an ordinary Lav, but not by a Lav which carries with it Kares (such as "Mechalelehah Mos Yumas").

(b)We counter that an Aseh anyway overrides a Lav, in spite of the fact that the Lav is more stringent than it (so what difference does it make if the Lav carries with it Kares as well?) What makes a Lav more stringent than an Aseh - is the fact that it is punishable by Malkus, which an Aseh is not.

3)

(a)Although the Torah includes all Kodshim in the Kares of "Kol Ish Asher Yikrav ... el ha'Kodoshim Asher Yakdishu ... " (in Parshas Emor), it nevertheless finds it necessary to add the Pasuk in Tzav "v'ha'Nefesh Asher Tochal Besar Zevach ha'Shelamim v'Tum'aso Alav ... v'Nichresah" - to teach us that a Tamei person is only Chayav Kares for eating Kodshei Mizbe'ach (such as Shelamim), but not Kodshei Bedek ha'Bayis (which do not go on the Mizbe'ach).

(b)The principle that governs this Derashah is - the eighth of Rebbi Yishmael's thirteen principles: 'Any P'rat (detail of a Klal) that was included in the Klal (the general category), and was then taken out to teach a new Chidush, that Chidush is not confined to the Prat, but reflects on the entire Klal'.

(c)Still searching for a source to include Achos Ishah in the Heter of Yibum (for the need to preclude her from "Alehah"), we now attempt to use this principle as the source - by postulating that Achos Achiv was included in the Arayos, and once the Torah takes it out of the Klal to teach us the Heter of Yibum, that Heter should extend to all the Arayos.

(d)We reject that suggestion however - on the grounds that in the principle of Rebbi Yishmael, both the Klal and the Prat are teaching us a Chumra (a Chiyuv Kares for eating them when one is Tamei); whereas in our case, we are trying to use Achos Achiv to superimpose a Kula on the Arayos.

4)

(a)So, based on another Beraisa, we try to learn Achos Ishto from another of the thirteen principles of Rebbi Yishmael.The Torah writes in Metzora "v'Shachat es ha'Keves bi'M'kom Asher Yishchat es ha'Chatas v'es ha'Olah bi'Mekom ha'Kodesh, Ki ka'Chatas ha'Asham". We do not need "Ki ka'Chatas ha'Asham" to designate ...

1. ... the location of the Shechitah of the Asham - because we know that already from "bi'Mekom Asher Yishchat es ha'Chatas" mentioned earlier.

2. ... the way the blood was to be sprinkled or how it was to be eaten - because we know that from "Zos Toras ha'Asham" in Tzav (which teaches us that all the Ashamos have the same Din).

(b)The Hekesh comes to teach us - that even though some of the blood of this particular Asham had the distinction of being placed on the owner's right thumb and right big toe, it nevertheless required sprinkling like the Chatas.

(c)We can deduce from here - that if not for the Hekesh, the blood of the Asham Metzora would not require sprinkling, which in turn, teaches us that something that was originally part of a Klal, and was then taken out to teach something new, cannot be returned to the Klal unless the Torah specifically reinstates it.

(d)In that case, we ought to apply the same principle to Eishes Achiv, and say - that Eishes Achiv has been given the Mitzvah of Yibum (the Heter to 'marry' an Ervah - a Chidush), but that that Heter does not automatically extend to the rest of the Klal. So why do we need "Alehah" (to preclude Achos Ishto from Yibum)?

5)

(a)We conclude that Achos Ishto ought to be included in the Mitzvah of Yibum - from a 'Mah Matzinu' from Eishes Achiv (seeing as both are Arayos).

(b)To deal with the problem that, whereas Eishes Achiv constitutes only one Isur, Achos Ishto constitutes two - we cite the principle 'Ho'il v'Ishteri, Ishteri'.

6)

(a)And we prove it from a Beraisa which discusses a Metzora whose eighth day of Taharah falls on Erev Pesach and who Toveled after seeing Keri on that day - and which permits him to enter the Sha'ar Nikanor (which is part of Machaneh Leviyah), something which is forbidden to any other Tevul Yom.

(b)The Sha'ar Nikanor (leading from the Ezras Nashim to the Ezras Yisrael) - belongs to Machaneh Leviyah.

(c)The Chachamim deliberately refrained from sanctifying it with the Kedushah of Machaneh Shechinah (like the Ezras Yisrael) - to enable the Metzora to enter it, in order to perform his purifying ceremony there.

(d)The Chachamim granted him this concession - because an Aseh which carries with it Kares (i.e. bringing of the Korban Pesach) overrides an Aseh which does not (i.e. a Tevul Yom entering the Har ha'Bayis).

(e)Rebbi Yochanan learns from the Pasuk in Divrei ha'Yamim "va'Ya'amod Yehoshafat bi'Kehal Yehudah Lifnei ha'Chatzar ha'Chadashah" - that min ha'Torah, a Tevul Yom is permitted to enter the Har ha'Bayis, and it was Yehoshafat who had just issued a new Takanah forbidding him to do so.

7)

(a)The Metzora stands in the Sha'ar Nikanor - for the purpose of placing his right hand, his right foot and his right ear inside the Ezras Yisrael, for the blood of his Asham and the Lug of oil to be placed on them as part of his purification ceremony.

(b)The Sugya in Zevachim asks how this is possible, according to Ula, who holds that 'Bi'ah b'Miktzas Shemah Bi'ah' (a partial entry is considered entry) - seeing as a Tevul Yom who enters the Machaneh Shechinah is Chayav Kares.

(c)Ula answers - that since the Torah permits a Metzora to 'enter' for his purification ceremony, the fact that he is a Ba'al Keri will not make any difference ...

(d)...a support for the principle 'Ho'il v'Ishteri Ishteri'.

(e)By the same token, we conclude - all the Arayos ought to be included in the Mitzvah of Yibum (because since Yibum permits the Isur of Eishes Achiv, it will not make any difference if she is also one of the other Arayos); and that explains why "Alehah" is required to forbid them.