Yevamot 10A

וסימניך: מת, נולד, ויבם. מת, נולד, ויבם.

And your mnemonic to remember how this might occur is as follows: Died, was born, entered into levirate marriage, died, was born, entered into levirate marriage. In other words, there were several brothers, and one, Reuven, died, before another, Yissakhar, was born, and another brother, Shimon, entered into levirate marriage with the deceased’s wife, and there was another brother, Levi, who died as well, and yet another brother, Zevulun, was born, and a 6th brother, Yehuda, entered into levirate marriage with the deceased’s wife. In this case, if Shimon and Yehuda were to die childless, their wives would come before the surviving brothers, Yissakhar and Zevulun, for levirate marriage. To one of them, Yissakhar, the wife of the first brother, Reuven, is prohibited, and for the other brother, Zevulun, the wife of the second brother, Levi, is likewise prohibited. This is an example of the type of case analyzed in the third chapter, in which the ruling is that the woman who is forbidden to this brother is permitted to that one, and she who is forbidden to this one is permitted to that one. In any case, R' Ḥiyya incorporates the opinion of R' Shimon into this principle, as he maintains that this scenario can apply to each case stated in the Mishnah. If so, the Mishnah evidently does deal with disputed legal rulings. How, then, can R' Yehuda HaNasi claim that the cases of the Mishnah are unanimous?

אלא לרבי הני כללי לית ליה. רב אדא קרחינא קמיה דרב כהנא אמר משמיה דרבא: לעולם אית ליה לרבי הני כללי, והכי קאמר ליה: אמו אנוסת אביו ⁠— בחדא משכחת לה, בתרתי לא קמשכחת לה.

§ Rather, R' Yehuda HaNasi does not hold in accordance with these general statements applied by R' Ḥiyya, and therefore he maintains that no issue subject to dispute was taught in the Mishnah. Rav Adda Karḥina, who was sitting before Rav Kahana, said in the name of Rava: Actually, one can explain that R' does hold in accordance with these general statements laid out by R' Ḥiyya. And this is what he said to Levi: In the case of his mother who had been raped by his father, you find this case with only one of the women; however, you cannot find it with two. In other words, the terms mentioned by R' Ḥiyya above: She who is forbidden to this one is permitted to that one, and: A sister who is a sister-in-law performs either ḥalitza or levirate marriage, apply only partly to this complex case.