Yevamot

In the case of a nissu'in widow they both agree  that it does not exempt, since no positive precept may override a combination of a positive and a negative precept.  They differ, however, in the case of an erusin widow. He who maintains that it  exempts [does so because] a positive precept supersedes a negative one; and he who maintains that it  does not exempt holds that the positive precept here does not supersede the negative one since [in this case] halizah is possible. An objection was raised: If they  had intercourse [with any of the forbidden women] they acquire [her as wife]!  -This is indeed a refutation. May this  be assumed to provide a refutation of the view of Resh Lakish also?  -Resh Lakish can answer you: I said it only in the case where the precept is fulfilled; here, however, halizah as a substitute for the levirate marriage is not a fulfilment of the precept. Raba said: Where in the Torah may an allusion be found to [the prohibition of] relations in the second degree?  It is said, For all these abominations have the men of the land done;  the expression, these  implies grave abominations, from which it may be inferred that there are milder ones. And what are these? The cases of incest of the second degree. What proof is there that 'these'  is an expression of gravity? — Because it is written in the Scriptures, And the mighty  of the land he took away.  May it be assumed that this view  differs from that of R. Levi? For R. Levi said: The punishments for [false] measures are more rigorous than those for [marrying] forbidden relatives; for in the latter case the word used is El,  but in the former Eleh.  — El implies rigour, but Eleh implies greater rigour than El.  Is not Eleh written also In connection with forbidden relatives?  -That [Eleh has been written] to exclude [the sin of false] measures from the penalty of kareth.  In what respect, then,  are they  more rigorous? — In the case of the former,  repentance is possible; in that of the latter  repentance is impossible. Rab Judah said: It  may be derived from the following: Yea he pondered, and sought out, and set in order many proverbs,  in relation to which 'Ulla said in the name of R. Eleazar, 'Before Solomon appeared, the Torah was like a basket without handles; when  Solomon came he affixed handles  to it. R. Oshaia said: It  may be derived from the following: Avoid it, pass not by it; turn from it, and pass on. Said R. Ashi: R. Oshaia's interpretation may be represented by the simile  of a man who guards an orchard. If he guards it from without, all of it is protected. If, however, he guards it from within, only that, section in front of him is protected but that which is behind him is not protected. This statement of R. Ashi, however, is mere fiction.  There,  the section in front of him, at least, is protected; while here were it not for the prohibition of incest of the second degree, one would have encroached upon the very domain of incest. R. Kahana said, it may be derived from here: Therefore shall ye keep My charge,  provide a charge to my charge. Said Abaye to R. Joseph: This,  surely, is Pentateuchal!  — It is Pentateuchal' but the Rabbis have expounded it.  All the Torah, surely- was expounded by the Rabbis!  But [the fact is that the prohibition  is] Rabbinical, while the Scriptural text is [adduced as] a mere prop. Our Rabbis taught: Who are the forbidden relatives in the second degree?  — His mother's mother, his father's mother, his father's father's wife, his mother's fathers wife, the wife of his father's maternal brother, the wife of his mother's paternal brother, the daughter-in-law of his son daughter-in-law his daughter. A man is permitted to marry the wife of his father-in-law and the wife of his step-son but is forbidden to marry the daughter of his step-son. His step-son is permitted to marry his  wife and his  daughter. The wife of his step-son may say to him, 'I am permitted to you though daughter is forbidden to you'. Is not the daughter of, his step-son forbidden, it being written in the Scriptures, Her son's daughter or her daughters daughter?  — As he wished to state in the latter clause, 'The wife of his step-son may say to him, "I am permitted to you though my daughter is forbidden to you", and though my daughter is forbidden to you Pentateuchally the Rabbis did not forbid me as a preventive measure', he stated in the previous clause also 'the daughter of his step-son'. If so,  could not the wife of his father-In-law also say, 'I am permitted to you and my daughter is forbidden to you', since she is his wife's sister?  -The prohibition of the one  is permanent;  that of the other is not. Rab said: Four [categories of] women [forbidden in the second degree] are subject to a limitation.  Of these Rab knew  three: The wife of a mother's  paternal brother, the wife of a father's  maternal brother, and one's daughter-in-law.  Ze'iri, however, adds also the wife of his mother's father. Said R. Nahman b. Isaac: Your mnemonic sign is, 'Above that of Rab'.  Why does not Rab include it?  — Because she  might be mistaken for the wife of one's father's father.  And Ze'iri? — Thither  one usually goes,  but hither  one does not usually go. Is not the prohibition of one's daughter-in-law