Reference for Keritot 29:5

אמר רב נחמן בר יצחק

Maybe R'Johanan B'Nuri maintains his view only in the instance of our Mishnah, because the prohibitions are at least of different designations; for she may be described as his mother-in-law and also as the mother of his mother-in-law and the mother of his father-in-law.

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