Yevamot

And your mnemonic is, 'Died, born, and performed the levirate marriage; died, born, and performed the levirate marriage'!  — Rabbi  does not accept these rules. R. Adda Karhina stated before R. Kahana in the name of Raba: Rabbi, in fact, does accept these rules,  but it was this that he meant to say to [Levi]:  [The application of the statement  to] a woman outraged by one's father is possible only in one [of its parts]; it is impossible, however, to apply it in [both its parts], for if Jacob outraged his two sisters,  it is possible [to apply that part of the statement relating to] 'her sister who is her sister-in-law',  but not that of 'she who is forbidden to one brother may be permitted to the other';  and if be outraged two strangers,  it is possible [to apply the statement], 'she who is forbidden to one brother may be permitted to the other'  but not that of 'her sister who is her sister-in-law'. R. Ashi said: Rabbi, in fact, does not accept these rules  and [our Mishnah] does deal with matters in dispute, and as to the meaning  of 'It seems to me that this man has no brains in his head' which he  addressed to him,  what he meant was this: 'Why did you not carefully consider our Mishnah? For our Mishnah represents the view of R. Judah who forbids the marriage of a woman that was outraged by one's father,  as it was taught: Six forbidden relatives come under greater restrictions,  since they are to be married to strangers only,  and their rivals are permitted.  [These are:] his mother, his father's wife and his father's sister [etc.].  Now, what is meant by "his mother"? If it be assumed to mean one who was legally married to his father, such a woman surely is "his father's wife".  Must it not consequently mean one who was outraged by his father? And yet it was stated, "since they are to be married to strangers only", implying "to strangers only but not to the brothers". Now, who has been heard to hold such an opinion? Surely it was R. Judah who forbids marriage with a woman who was outraged by one's father.  Hence  it was not included in our Mishnah.' Said Rabina to R. Ashi: [Such a levirate relationship]  is possible even according to R. Judah if and when one had married  illegally!  — The author of the Mishnah is not concerned with an 'if'.  Said R. Ashi to R. Kahana: This  is also possible without the 'if',  where Jacob  outraged his daughter-in-law, begat from her a son, and then Reuben  died without issue, and she thus came into levirate relationship with her son;  and since she is forbidden to him,  her rival also is likewise forbidden!  — The other replied: [The author of our Mishnah] deals only with lawful brotherhood but not with brotherhood which is due to a forbidden act. Levi nevertheless  inserted it  in his Mishnah. For Levi taught: One's mother sometimes exempts her rival  and sometimes she does not exempt her. If his mother, for instance,  was lawfully married to his father, and then she was married  to his paternal brother  who subsequently died, such a mother does not exempt her rival.