Since the Torah concludes "Ad Olam", why does it insert "Gam Dor Asiri"?
1.
Sifri: To teach us via a Gezeirah Shavah "Dor Asiri" "Dor Asiri" from Amoni u'Mo'avi - in the following Pasuk - that a Mamzer, about which the Torah writes only "Gam Dor Asiri" is also forbidden forever.
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See Torah Temimah, note 22 as to wwhy the Torah declines to write "ad Olam" by Mamzer.
2)
What is the Din regarding an Amonis and a Mo'avis?
1.
Yerushalmi Yevamos 8:3: We Darshen "Amoni", 've'Lo Amonis', "Mo'avi", 've'Lo Mo'avis' - because it is not the way of women to hire
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and to plan
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(or to provide food to travelers in the desert
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- Yevamos, 73a).
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