Reference for Bekhorot 94:28

מסייע ליה לרבי ינאי דא"ר ינאי

but if they were not identified in the beginning and in the end became mixed, they cannot write out a power of attorney to each other.<span class="x" onmousemove="('comment',' And the Mishnah here also refers to a case where the children who became mixed were never originally identified as to who was the first-born, so that no-one acquired any claim on the estate as a first-born. This therefore confirms the opinion of R. Jannai, v. B.B. 127a.');">¹⁹</span>

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