Reference for Yevamot 5:8

ומאי איריא דתנא פוטרות ליתני אוסרות אי תנא אוסרות הוה אמינא אסור לייבם אבל מיחלץ חלצה קמ"ל

the expression EXEMPT<span class="x" onmousemove="('comment',' Which might imply that the levirate marriage in these cases is not obligatory but optional. ');">¹⁹</span> and not that of<span class="x" onmousemove="('comment',' v. supra p. 4, n. 13. ');">²⁰</span> 'prohibit'?<span class="x" onmousemove="('comment',' Since, in fact, no marriage with a deceased brother's widow is permitted whenever the obligation of the levirate marriage does not exist. ');">²¹</span>

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