Reference for Bekhorot 50:45

ור' אסי מאי טעמא לא אמר כרבי חייא בר אבא

Why does not R'Assi concur with [the interpretation of] R'Hiyya B'Abba? - He can reply [as follows]: This would indeed hold good with regard to levitical uncleanness,<span class="x" onmousemove="('comment',' The Rabbis maintaining the he digs until he reaches a rock etc. and this is sufficient, for although he does not find it, one may say that it was removed.');">²¹</span>

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