Yevamot

— [They were intended] to exclude the respective rulings of Rab and R. Assi.  What, [however, do the numerals] exclude according to Rab and R. Assi? — If they share each other's views, one numeral would serve to exclude the rival of one who made a declaration of refusal,  and the other to exclude the rival of a wife whom [her husband] remarried after having divorced her.  If they do not share the views of each other, [each would regard] one [numeral as serving] to exclude the ruling of his colleague;  and the other numeral, as serving to exclude either the rival of one who made a declaration of refusal  or the rival of a wife whom [her husband] remarried after having divorced her. According to Rab and R. Assi these  should have been enumerated in our Mishnah! — [This could not be done] because the law of the rival's rival  is not applicable [to these cases]. Whence is this law  derived?  — [From] what our Rabbis taught: And thou shalt not take a woman to her sister, to be a rival to her, to uncover her nakedness, 'aleha [beside her] in her lifetime,  what need was there for the expression ''aleha'?  Because it was stated, Her husband's brother shall go in 'aleha [unto her],  it might have been imagined  that Scripture  speaks even of any of all the forbidden relatives enumerated in the Torah. Hence it was here  stated, ''aleha'  and elsewhere  it was also stated ''aleha'.  Just as elsewhere it is in the case of a precept  so here also it is in the case of a precept;  and yet did not the All Merciful say, Thou shalt not take.  We are thus in a position to know the law concerning herself;  whence do we derive the law concerning her rival? — From the Scriptural expression, To be a rival to her.  We have so far deduced the law concerning her rival only. Whence do we arrive at the law concerning her rival's rival? — From the fact that Scripture uses the expression li-zeror  and not that of la-zor.  Thus we have deduced the law concerning a wife's sister, whence is the law concerning the other forbidden relatives to be inferred? — It can be answered: As a wife's sister is singled out in that she is a forbidden relative, the penalty for presumptuous intercourse with her is kareth  and for unwitting intercourse a sin-offering, and she is forbidden to the levir, so also any woman who is a forbidden relative, and the penalty for presumptuous intercourse with whom is kareth  and for unwitting intercourse a sin-offering, is forbidden to the levir. Now we know the law concerning themselves only;  whence is the law con cerning their rivals deduced? — It may be answered: As a wife's sister is singled out in that she is a forbidden relative, kareth is incurred by presumptuous intercourse with her and a sin-offering for unwitting intercourse, and she is forbidden to the levir, and her rival is forbidden, so also in the case of any woman who is a forbidden relative, and for presumptuous intercourse with whom is incurred the penalty of kareth and for unwitting intercourse a sin-offering, and who is forbidden to the levir, her rival is forbidden. Hence have the Sages said: FIFTEEN [CATEGORIES OF] WOMEN EXEMPT THEIR RIVALS AND THEIR RIVALS' RIVALS, AND SO ON, AD INFINITUM, FROM THE HALIZAH AND FROM THE LEVIRATE MARRIAGE. One might assume that the six more rigidly forbidden relatives  are also included in the ruling,  so that their rivals also  are forbidden,  hence it must be stated:  As a wife's sister is singled out in that she is a forbidden relative, kareth is incurred for presumptuous intercourse with her and a sin-offering for unwitting intercourse, she may be married to the other brothers, but is forbidden to the levir, and her rival is forbidden, so also in the case of any woman who is a forbidden relative, for presumptuous intercourse with whom is incurred the penalty of kareth and for unwitting intercourse a sin-offering, who may marry one of the other brothers, but is forbidden to the levir, her rival also is forbidden; excluded, however, are the six more rigidly forbidden relatives. Since they may not be married to the other brothers, their rivals are permitted; for [the law of] 'rival'  is applicable only [to widows] of a brother. Thus we have deduced the prohibition. Whence, however, is the penalty inferred? — Scripture said, For whosoever shall do any of these abominations etc. [shall be cut off from among their people.] The reason,  then, is because the All Merciful has written, ''aleha',  otherwise it would have been said that levirate marriage may be contracted with the wife's sister; what is the reason? Is it because we assume that a positive precept,  supersedes a negative precept?  Surely, it is possible that  the rule that a positive precept supersedes a negative precept applies only where the latter is a mere prohibition; does it, however, supersede a prohibition involving the penalty of kareth?  Furthermore, whence is it derived that it may supersede even a mere prohibition?