Yevamot 52A

האי גט אחר גט הוא? אמר רב יהודה, הכי קאמר: גט אחר הגט, ומאמר אחר מאמר ⁠— כדאמרן, יבם אחד ויבמה אחת כיצד התרתן? עשה מאמר ביבמתו ונתן לה גט ⁠— צריכה הימנו חליצה.

The Talmud is puzzled: Is this case upon which the Mishnah elaborates the case of a bill of divorce after a bill of divorce that is mentioned first in the Mishnah? The Mishnah first is referring to a bill of divorce after a bill of divorce, but when it comes to the specifics, it mentions a bill of divorce after levirate betrothal. Rav Yehuda said: This is what the tanna is saying: With regard to a bill of divorce after a bill of divorce, and levirate betrothal after levirate betrothal, it is as we said and these cases were explained in the baraita, but with regard to one yavam and one yevama, how is their complex bond released? He then proceeds to delineate: If he performed levirate betrothal with his yevama and gave her a bill of divorce, she requires ḥalitza from him.

עשה מאמר ובעל ⁠— הרי זו כמצותה. לימא מסייע ליה לרב הונא, דאמר רב הונא: מצות יבמה ⁠— מקדש ואחר כך בועל. אימא: אף זה כמצותה.

§ The Mishnah teaches: If the yavam performed levirate betrothal and had sex, this is done in accordance with its mitzva. The Talmud suggests: Let us say that this Mishnah supports the statement of Rav Huna. As Rav Huna said: The mitzva of a yevama is properly performed when the yavam betroths the yevama and only afterward has sex. This statement indicates that the levirate betrothal is a necessary component of the mitzva, and the Mishnah seems to imply the same. The Talmud refutes this claim: This is not necessarily the case, as you can read the Mishnah as stating that this also is in accordance with its mitzva. If he performed levirate betrothal and then had sex, this too is a proper manner to proceed, but we do not have to interpret the Mishnah as indicating that this is the only way to fulfill the mitzva.