Reference for Yevamot 204:2

אמר רב יהודה אמר רב יבמה שהגדילה בין האחין מותרת לינשא לאחד מן האחין ואין חוששין שמא חלצה סנדל לאחד מהן טעמא דלא חזינן הא חזינן חיישינן

The reason, then<span class="x" onmousemove="('comment',' Why halizah is not apprehended. ');">⁴</span> is because we did not actually observe it,<span class="x" onmousemove="('comment',' That she drew off the sandal from the foot of any brother. ');">⁵</span> had we, however, observed it<span class="x" onmousemove="('comment',' That she drew off the sandal from the foot of any brother. ');">⁵</span> the possibility [that her <i>halizah</i> was valid] would have had to be taken into consideration.<span class="x" onmousemove="('comment',' And the sister-in-law would be forbidden to marry any of the brothers. ');">⁶</span>

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