When trying to figure out how to add havdalah to havineinu (which would allow havineinu to be said on Motzei Shabbos) the gemara cites a mishna which gives opinions as to where to place havdalah. R. Akiva says to make it its own bracha. This is rejected. But R. Eliezer says to add it into Modim. If this were to be done, havdalah would have a natural place in havineinu, and yet, the gemara never deals with this option! It explains why we don;t follow R. Akiva and then moves to Mar Zutra's point.
Why isn't R. Eliezer's position considered? It would solve everything.
Dan, Teaneck, NJ
Rabeinu Yonah explains that the reason that we rule like Shmuel (who says that one cannot say Havineinu on Motzei Shabbos because Havdalah cannot be inserted into Havineinu), despite other prominent opinions in this Gemara, is that the Gemara understands that the strict rule for Havineinu is that we only include a shortening of a Berachah in Havineinu, not a short version of something that is added to a Berachah.
All the best,
Yaakov Montrose