Yevamot

Between the one  who was given  a letter of divorce and the other  to whom a ma'amar had been addressed  who is to be preferred?  Is she who was divorced to be preferred.  or is, perhaps, she to whom the ma'amar had been addressed to be preferred since she is nearer to him in respect to intercourse? — R. Ashi replied, Come and hear: R. Gamaliel, however, admits  that a letter of divorce  after a ma'amar,  and a ma'amar  after a letter of divorce  is valid.  Now, if a letter of divorce has the preference.  the ma'amar after it should have no validity; and if the ma'amar has the preference, the divorce after it should have no validity. Consequently it must be concluded that they have both equal validity. This proves it. R. Huna  said in the name of Rab: If two sisters who were sisters-in-law became subject to one levir, the one is permitted  when he  has participated in her halizah; and the other is permitted  when he has participated in her halizah. If the first  died  he  is permitted [to marry] the second,  and there is no need to state that if the second  died  the first is permitted,  since, as a sister-in-law who was permitted,  then forbidden  and then again permitted,  she returns to her former state of permissibility. R. Johanan, however, said: If the second  died  he  is permitted to marry the first,  but if the first  died he is forbidden to marry the second.  What is the reason? Because any sister-in-law to whom the injunction. Her husband's brother shall go in unto her  cannot be applied at the time of her coming under the obligation of the levirate marriage  is, indeed,  like the wife of a brother who has children and is, consequently, forbidden. But does not Rab hold the same view?  Surely Rab said: Any woman to whom the injunction, Her husband's brother should go in unto her  cannot be applied at the time of her coming under the obligation of the levirate marriage is, indeed, like the wife of a brother who has children and is, consequently, forbidden!  -That statement  applies only to the case where the woman is faced with the prohibition of 'a wife's sister', which is Pentateuchal;  here, however, [the prohibition due to] the levirate bond is only Rabbinical. R. Jose b. Hanina raised the following objection against R. Johanan:  IN THE CASE OF FOUR BROTHERS, TWO OF WHOM WERE MARRIED TO TWO SISTERS, IF THOSE WHO WERE MARRIED TO THE SISTERS DIED, BEHOLD, THESE MUST PERFORM HALIZAH BUT MAY NOT BE TAKEN IN LEVIRATE MARRIAGE. But why? Let one of the brothers take on the duty of participating in the halizah with the second widow, and thus place the first widow, in relation to the second, in the category of a deceased brother's wife that was permitted- then forbidden, and then again permitted,  and thus she would return to her former state of permissibility! — The other replied: I do not know who was the author of the statement concerning the sisters.  But let him rather reply that the meaning of the expression of MUST PERFORM THE HALIZAH, which had been used, indeed signifies that only one is to perform the halizah!  -The expression used was THEY MUST PERFORM THE HALIZAH.  Then let him reply that the expressions THEY MUST PERFORM THE HALIZAH  refers to women generally  who perform the halizah!-It was stated, BEHOLD THESE.  Let him, then, reply that [this is a case] where halizah was already performed by the first!  -[The expression] THESE MUST PERFORM HALIZAH