Yevamot

What is R. Gamaliel's reason?  — Because he was in doubt whether a letter of divorce does, or does not set aside [the levirate bond, and whether] a ma'amar does, or does not effect a kinyan.  'Whether a letter of divorce does, or does not set aside the levirate bond': If the first  does set aside [the levirate bond], what purpose could the latter serve?  If the first  does not set aside [the levirate bond], the latter also does not set it aside. 'Whether a ma'amar does, or does not effect a kinyan': if the first  does effect a kinyan, what purpose could the latter serve?  And if the first  effects no kinyan, the latter also does not. Abaye raised the following objection against him: R. Gamaliel, however, admits that 'there is [validity in] a letter of divorce after a ma'amar,  in a ma'amar after a letter of divorce,  in a letter of divorce after cohabitation and a ma'amar,  and in a ma'amar after cohabitation and a letter of divorce'.  Now, if R. Gamaliel was in doubt,  the cohabitation  should be regarded as if it had taken place at the beginning,  and thus constitute a kinyan; for surely we have learnt, IN THE CASE OF COHABITATION, IF IT TOOK PLACE FIRST, NO ACT THAT FOLLOWS IT HAS ANY VALIDITY! But, said Abaye, though obvious to R. Gamaliel that a letter of divorce does set aside the levirate bond and that a ma'amar does effect a kinyan,  the Rabbis have nevertheless ruled that with the sister-in-law a letter of divorce is partially valid and a ma'amar is partially valid. Consequently, a letter of divorce after another letter of divorce does not set aside the levirate bond since this  was already set aside by the first,  and a ma'amar after a ma'amar does not constitute a kinyan since this kinyan  has already been constituted by the first;  with a letter of divorce after a ma'amar, and a ma'amar after a letter of divorce, however, the one act  sets aside  while the other  effects a kinyan.  (And the Rabbis?  — [They hold that] the Rabbis have instituted for every levir a letter of divorce and a ma'amar in respect of every sister-in-law.) But as to an invalid cohabitation  [according to R. Gamaliel]  it is [in one respect] of superior force to a ma'amar and [in another respect] of inferior force to a ma'amar. It is superior to a ma'amar, since whereas a ma'amar after another ma'amar is not effective,  an act of cohabitation after a ma'amar is effective.  It is inferior to a ma'amar, for whereas a ma'amar after a letter of divorce constitutes a kinyan of all that the letter of divorce has left,  cohabitation after a letter of divorce does not constitute a kinyan of all that the divorce has left. Our Rabbis taught; How [are we to understand] R. Gamaliel's statement that there is [no validity in] a letter of divorce after another letter of divorce? If two sisters-in-law have fallen to the lot of one levir, and he gave a letter of divorce to one as well as to the other, he submits, in accordance with R. Gamaliel's statement, to halizah from the first,  and is forbidden to marry her relatives,  though the relatives of the second one are permitted to him.  But the Sages said: If he gave a letter of divorce to one and to the other, he is forbidden to marry the relatives of both  and he submits to halizah from either of them. And the same law applies where there are two Ievirs  and one sister-in-law. What did R. Gamaliel mean by his statement that  there is no [validity in] a ma'amar after another ma'amar'? If two sisters-in-law have fallen to the lot of one levir, and he addressed a ma'amar to the one as well as to the other, he gives, according to R. Gamaliel, a letter of divorce to the first, submits also to her halizah, and is in consequence forbidden to marry her relatives,  though the relatives of the second are permitted to him.  The Sages, however, said: He gives letters of divorce to both, and the relatives of both are forbidden to him,  while he submits to halizah from one of them. And the same law is to be applied where there are two levirs and one sister-in-law. The Master said, 'If he gave  a letter of divorce to one as well as to the other, he submits, according to R. Gamaliel's statement, to halizah from the first and is forbidden to marry her relatives, though the relatives of the second are permitted to him'. Must this be assumed to present an objection against a ruling of Samuel, since Samuel stated, 'If he submitted to halizah from the one  who had been divorced,  her rival  is not thereby exempt'!  — Samuel can answer you: What I said was in agreement with him who maintains that a levirate bond exists,  while R. Gamaliel holds the opinion that no levirate bond exists. Since R. Gamaliel, however, is of the opinion that no levirate bond exists,