I was on my way to the sea and it occurred to me what תוספות ראש השנה י''ג ע''א means. In fact, it is true that תוספות is saying that the גמרא in עבודה זרה כ''ג ע''ב is referring to the   אשרות שמדורות הראשונים in the question of the גמרא, not the answer. For like רב שך is saying, the answer refers to the trees that were on the land at the time it was given to Avraham. The thing that made this a little confusing is that the גמרא itself qualifies the question after it has already given the answer. That is it asks, "Why did Israel have to burn the אשורת? After all, no one can cause to be forbidden that which does not belong to him. And you can not answer "It refers to the אשרות that were planted after the land was given to Avraham and those trees in fact belonged to the Canaanites," because ביטול nullification would have been enough. So then the גמרא answers that it refers to trees that were in the land at the time of Avraham, and then those trees were worshiped after Israel worshiped the golden calf, and thus those trees would have been required to be burnt.] This is why תוספות refers to the "אשרות שמדורות הראשונים" since he is talking about the question of the גמרא, not its answer.

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