In Insights to the Daf-

>>1) DERIVING A PROHIBITION THROUGH "SEMUCHIN"

QUESTION: Rebbi Yehudah maintains that "Semuchin" ...

Why, then, when the Gemara explains the view of Rebbi Yehudah, does it say that the second half of the verse is superfluous? It is clear from the view of the Rabanan that it is not superfluous -- it teaches the prohibition of "Shomeres Yavam" of one's father. Since that part of the verse is not extra, Rebbi Yehudah should not be able to apply Semuchin to derive the prohibition of "Anusas Aviv." (TOSFOS DH Im Ken)

ANSWERS:

(a) TOSFOS explains ...

(b) The RIVAN answers that there is another reason for why Rebbi Yehudah does not accept that the verse refers to "Shomeres Yavam" of one's father. In the case of the "Shomeres Yavam" of one's father, the woman who is bound to do Yibum with his father (whose brother died childless) is already forbidden to him by two other prohibitions: the prohibition against a Yevamah marrying anyone other than her husband's brother ("Isur Yevamah la'Shuk"), and the prohibition of "Eshes Achi Aviv" (the wife of the brother of one's father). If this verse would refer to the "Shomeres Yavam" of one's father, then it would teach a third prohibition for an act which is already forbidden by two other prohibitions. It is illogical to assume that the intent of the verse is to add a third Lo Ta'aseh to one act. (Although the first half of the verse adds a Lo Ta'aseh to the prohibition of "Eshes Aviv," that is only a second, and not a third, Lo Ta'aseh.) Therefore, Rebbi Yehudah understands that the verse refers only to "Eshes Aviv," and, consequently, the second half of the verse is extra and is able to teach, by way of Semuchin, the prohibition of "Anusas Aviv." (This also appears to be the approach of Rashi (DH Lichtevei).)<<

Regarding the answer of the Rivan that you quoted on the Insights to the Daf for Yevamos 4, where is the Rivan that says the opinion and why does he not like the answer of Tosfos?

Oriel, United States