Yevamot

a perfect kinyan,  he taught us  that [the halachah is] not in accordance with Beth Shammai. But then as to R. Nahman's [inference] does not that of R. Ashi present a difficulty? And should you reply that the same law, that her rival is permitted,  is also applicable to the case where he  died first and the other brother  divorced his wife afterwards,  what [it could be objected] would THIS IS exclude? It might exclude the case where he  married her  first and then divorced his wife.  This might be a satisfactory explanation if he  holds the view of R. Jeremiah who said, 'Break it up: He who taught the one did not teach the other,'  [for, if this is so]. one Tanna may hold the opinion that it is death  that causes the subjection  while the other might be of the opinion that it is the original marriage  that causes the subjection,  and THIS IS would thus exclude the case where he first married  and then divorced;  if, however, he is of the same opinion as Raba who said, 'Both statements may in fact represent the views of one Tanna, it being a case of "this and there is no need to state that"',  what does THIS IS exclude?  — He  has no alternative but to adopt the view of R. Jeremiah. And according to Raba,  the explanation would be satisfactory if he held the View of R. Ashi,  for then, THIS IS would exclude the case of one who died without first divorcing his wife;  if, however, he holds the same view as R. Nahman,  what would THIS IS exclude?  -He  has no alternative but to accept the view of R. Ashi. MISHNAH. [IF IN THE CASE OF ANY ONE OF] ALL THESE  THE BETROTHAL OR DIVORCE  WAS IN DOUBT, BEHOLD, THESE RIVALS MUST PERFORM THE HALIZAH  BUT MAY NOT BE TAKEN IN LEVIRATE MARRIAGE.  WHAT IS MEANT BY DOUBTFUL BETROTHAL? IF WHEN HE THREW TO HER A TOKEN OF BETROTHAL  IT WAS UNCERTAIN WHETHER IT FELL NEARER TO HIM  OR NEARER TO HER,  THIS IS A CASE OF DOUBTFUL BETROTHAL. DOUBTFUL DIVORCE? IF HE WROTE A LETTER OF DIVORCE IN HIS OWN HANDWRITING AND IT BORE NO SIGNATURES OF WITNESSES,  OR  IF IT BORE SIGNATURES BUT NO DATE, OR IF IT BORE A DATE BUT THE SIGNATURE OF ONLY ONE WITNESS, THIS IS A CASE OF DOUBTFUL DIVORCE. GEMARA. In the case of divorce, however, It is not stated IT WAS UNCERTAIN WHETHER IT FELL NEARER TO HIM OR NEARER TO HER; what is the reason?  -Rabbah replied: This woman  is in a state of permissibility to all men;  would you forbid her [marriage] because of a doubt?  You must not forbid her because of a doubt!  Said Abaye to him: If so, let us also in the matter of betrothal say: This woman  is in a state of permissibility to the levir;  would you forbid her  because of a doubt? You must not forbid her because of a doubt! — There  [it leads] to a restriction.  But it is a restriction which may lead to a relaxation! For, sometimes, he would betroth her sister  by betrothal that was not uncertain, or it might occur that another man would betroth her also by a betrothal that was not uncertain and, as the Master has forbidden her rival to be taken in levirate marriage. it would be assumed that the betrothal of the first  was valid and that that of the latter was not!