Ate During Twilight Before Rosh Chodesh

As explained in Orach Chaim 188:10, how is twilight defined? For example, if someone started eating before sunset but did not eat a kezayit before sunset, and only managed to eat a kezayit during twilight, is it considered that he started his meal on Shabbat, and therefore when he blesses Birkat Hamazon after dark, should he say 'Retze'? And what is the ruling if Rosh Chodesh falls on Motzei Shabbat and he ate a kezayit of bread only during twilight, and blesses Birkat Hamazon after dark, what should he mention?

Shalom Rav

In such a case, he should only say 'Retze', as if he ate on Shabbat itself.

According to the instructions of the Gaon, Rabbi Amram Fried, shlita

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