R. Tarfon opened and said: If she went and married the brother of the man to whom she had been forbidden (by the get), and he died without children, how could she become the yevamah of his brother? Would he (her first husband) not be making a condition against what is written in the Torah (i.e., the mitzvah of yibum)? And if anyone makes a condition against what is written in the Torah, his condition is not valid — so that this is not krithuth.
R. Yossi Haglili said: Where do we find in the Torah that she is permitted to one and forbidden to another? But if she is permitted to one, she is permitted to all men, and if she is forbidden to one, she is forbidden to all men — so that this is not krithuth.