Reference for Yevamot 168:2

הא ודאי משום סיפא משום דקבעי למיתנא סיפא כ"ג שנשא את האלמנה יש לו אח כ"ג או כהן הדיוט דוקא אלמנה אבל בתולה חזיא ליה משום הכי קתני אלמנה

behold, [it may be pointed out, the case of] the HALAL WHO MARRIED A WOMAN OF LEGITIMATE STATUS<span class="x" onmousemove="('comment',' Who becomes, thereby. disqualified from marrying his brother. ');">³</span> where it is not said that 'the original marriage causes the subjection'!<span class="x" onmousemove="('comment',' Had this been the case, his brother should have been permitted to marry her, owing to the fact that at the time of her marriage with the deceased (when she presumably became subject to the levirate marriage) she was no halalah. ');">⁴</span>

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