Rif Brachot 30b

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30b

{continuing Brachot 42b, though currently digressing to Pesachim 103b}
Amemar blessed over every single cup {of wine}.
Mar Zutra blessed over the first cup and the cup of blessing {Rashi: at the end of Birchat HaMazon, for Birchat HaMazon interrupted}.
Rav Ashi blessed on the first cup and made no further blessings.
Rav Acha said: I, like whom shall I conduct myself?
Amemar said: I changed my mind {each time, to drink a new cup}.
Mar Zutra said: I conduct myself like the students of Rav -

for Rav Brona and Rav Chananel the students of Rav were sitting at a meal. Rav Yeva the elder waited on them. They said to him: Give to us {Hav Lan}, and we will drink. {Rashi's girsa there - Give to us, and Let us bless. And then they [changed their minds and] wished to drink.} He said to them: so said Rav: Once you {plural} say Give to us and Let us bless, it is forbidden for you to drink.

And so is the halacha.
{
While in the gemara Rav Ashi says the halacha is NOT like the students of Rav, his proof, from a statement of Rav, is rejected.
What we are deriving from the students of Rav is that Birchat HaMazon removed his thoughts from the meal, and thus one needs to bless again on wine taken after the meal, after Birchat HaMazon.
}

{we now return to Brachot 42b}
Rav Yitzchak bar Yosef visited the house of Abaye on Yom Tov.
He saw that he {Abaye} blessed and then blessed again.
He said to him {Abaye}: Does not Master hold like the statement of Rabbi Yehoshua ben Levi - if he blessed on the wine before the meal, it exempts the wine after the meal?
He {Abaye} said to him: I changed my mind {to drink another cup}.

If wine was brought to them within the meal, this does not exempt the wine after the meal - for this {wine during the meal} is for steeping {the food in} and this {the wine after the meal} is for drinking - for we establish {the halacha} like the students of Rav who say it does not exempt.

MISHNA:

IF [THOSE AT THE TABLE] ARE SITTING UPRIGHT, EACH ONE SAYS GRACE FOR HIMSELF;
IF THEY HAVE RECLINED, ONE SAYS GRACE FOR ALL.

IF WINE IS BROUGHT TO THEM IN THE COURSE OF THE MEAL, EACH ONE SAYS A BENEDICTION FOR HIMSELF;
IF AFTER THE MEAL, ONE SAYS IT FOR ALL.

THE SAME ONE SAYS [THE BENEDICTION] OVER THE PERFUME, {continues on next page: ALTHOUGH THE PERFUME IS NOT BROUGHT IN TILL AFTER THE MEAL.}