Yevamot

refers to the years of the generations.  If one is worthy one is allowed to complete the full period; if unworthy. the number is reduced; so R. Akiba. But the Sages said: If one is worthy years are added to one's life;  if unworthy, the years of his life are reduced. They said to R. Akiba: Behold, Scripture says,  And I will add unto your days fifteen years!  He replied: The addition was made of his own,  You may know [that this is so]  since the prophet  stood up and prophesied: Behold, a son shall be born to the house of David, Josiah by name,  while Manasseh  had not yet been born.  And the Rabbis!  — Is it written 'from Hezekiah'? It is surely written, 'To the house of David';  he  might be born either from Hezekiah or from any other person. IF A MAN'S WIFE DIED etc. IF A MAN'S SISTER-IN-LAW DIED etc. R. Joseph said: Here Rabbi taught an unnecessary Mishnah. MISHNAH. R. GAMALIEL SAID: THERE IS NO [VALIDITY IN A] LETTER OF DIVORCE AFTER ANOTHER LETTER OF DIVORCE,  NOR IN A MA'AMAR AFTER ANOTHER MA'AMAR  NOR IN AN ACT OF COHABITATION AFTER ANOTHER ACT OF COHABITATION,  NOR IN A HALIZAH AFTER ANOTHER HALIZAH.  THE SAGES, HOWEVER, SAID: A LETTER OF DIVORCE HAS VALIDITY AFTER ANOTHER LETTER OF DIVORCE,  AND A MA'AMAR AFTER ANOTHER MA'AMAR;  BUT THERE IS NO VALIDITY IN ANY ACT AFTER COHABITATION OR HALIZAH. HOW [IS THE RELEASE FROM THE LEVIRATE BOND  EFFECTED]? — IF A LEVIR ADDRESSED A MA'AMAR TO HIS SISTER-IN-LAW AND SUBSEQUENTLY GAVE HER A LETTER OF DIVORCE, IT IS NECESSARY FOR HER TO PERFORM THE HALIZAH WITH HIM.  IF HE ADDRESSED TO HER A MA'AMAR AND PARTICIPATED IN THE HALIZAH, IT IS NECESSARY FOR HER TO OBTAIN FROM HIM A LETTER OF DIVORCE.  IF HE ADDRESSED TO HER A MA'AMAR AND THEN COHABITED WITH HER, BEHOLD THIS IS IN ACCORDANCE WITH THE PRESCRIBED PRECEPT. IF THE LEVIR GAVE HER A LETTER OF DIVORCE AND THEN ADDRESSED TO HER A MA'AMAR, IT IS NECESSARY FOR HER TO OBTAIN [A SECOND] LETTER OF DIVORCE  AND TO PERFORM THE HALIZAH.  IF HE GAVE HER A LETTER OF DIVORCE AND THEN COHABITED WITH HER, IT IS NECESSARY FOR HER TO OBTAIN A LETTER OF DIVORCE AND TO PERFORM THE HALIZAH.  IF HE GAVE HER A LETTER OF DIVORCE AND THEN SUBMITTED TO HALIZAH, THERE IS NO VALIDITY IN ANY ACT  AFTER HALIZAH HAD BEEN PERFORMED. IF THE LEVIR SUBMITTED TO HALIZAH AND THEN ADDRESSED TO HER A MA'AMAR, GAVE HER A LETTER OF DIVORCE, OR COHABITED WITH HER; OR IF HE COHABITED WITH HER AND THEN ADDRESSED TO HER A MA'AMAR, GAVE HER A LETTER OF DIVORCE, OR SUBMITTED TO HER HALIZAH, NO ACT IS VALID AFTER HALIZAH