Reference for Yevamot 206:7

א"ל אי הכי סנדל שאין שלו נמי אימר דאמור רבנן דיעבד לכתחלה מי אמור א"ל הכי קאמינא לך הב ליה ואקני ליה:

said to him,<span class="x" onmousemove="('comment',' Abaye. ');">²⁰</span> 'that the Rabbis spoke<span class="x" onmousemove="('comment',' In ruling that halizah with a left-foot sandal is valid. V. our Mishnah. ');">²¹</span> only of a fait accompli; did they, however, speak also of what is permissible ab initio?' The other<span class="x" onmousemove="('comment',' Abaye. ');">²⁰</span> replied: If so, in respect of a sandal that is not the levir's own, it might also be suggested that the Rabbis spoke<span class="x" onmousemove="('comment',' Cf. supra n. 4, mutatis mutandis. ');">²²</span>

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