Reference for Chullin 254:9

ואי דאין מעלין ארוכה טומאת נבלה נמי ליטמו

they should not be rendered unclean even In respect of food uncleanness, and if they cannot be restored they should be then rendered unclean also in respect of nebelah uncleanness! - In fact they cannot be restored, but with regard to nebelah Uncleanness it is different, for the Divine Law says.

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