Responsa for Yevamot 75:14

וקסבר עולא זיקת ארוסה עושה ספק ארוסה

in which there is no dispute between them<span class="x" onmousemove="('comment',' Beth Shammai and Beth Hillel. ');">²⁹</span> differ from the final clause<span class="x" onmousemove="('comment',' Where the widow had died. ');">³⁰</span> in which they<span class="x" onmousemove="('comment',' Beth Shammai and Beth Hillel. ');">²⁹</span> do dispute?<span class="x" onmousemove="('comment',' Why is the widow in the first case regarded as the confirmed possessor of the property and allowed to dispose of it in any manner she desires, while in the second case her right of possession is in dispute, her rightful heirs not being regarded as the lawful and undisputed successors to her property? ');">³¹</span> 'Ulla replied: The first clause deals with a woman who became subject to the levirate marriage<span class="x" onmousemove="('comment',' Lit., 'when she fell'. ');">³²</span> while betrothed, and the final clause with one who became subject to the levirate marriage<span class="x" onmousemove="('comment',' Lit., 'when she fell'. ');">³²</span> while married. And 'Ulla is of the opinion that the levirate bond<span class="x" onmousemove="('comment',' Between the widow and the levir, due to the obligations of the levirate. ');">³³</span> of a betrothed woman renders her 'doubtfully betrothed'<span class="x" onmousemove="('comment',' The levirate bond not carrying the same force as actual betrothal. ');">³⁴</span>

Teshuvot Maharam