Reference for Niddah 120:20

אמר ליה רב נחמן

both are unclean.<span class="x" onmousemove="('comment',' Since the other who sits behind her would naturally shift her position towards the mill and, assuming sometimes the same position as the inner one, would be as likely as she to be the cause of the stain in that spot. As it is thus uncertain which of the two was the cause both must be regarded as unclean. ');">²¹</span>

Jastrow

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