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What is "ve'ha'Ir asher ba'Nachal" referring to?
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Targum Yonsan: It refers to a small town that was built in the middle of the brook of Arnon. 1
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Oznayim la'Torah: Which the Torah does not name due to its small size and insignificance. And the Torah is teaching us that the brook of Arnon itself belonged to Sichon - since he captured it from Amon.