Chapter 4, Mishna 1, 2
Lost in Torah study, Nechemya wandered out of his Shabbos boundary. Rav Chisda pointed it out to Rav Nachman, who answered, build a human enclosure connecting him to his boundary so he can reenter. Rav Chisda hesitated. Rava and Rav Nachman bar Yitzhak speculate, what is Rav Chisda’s dilemma? Is he wondering whether Nechemya can roam within such an enclosure now that he’s limited to to his 4 cubits? Or whether he can reenter if the new enclosure doesn’t quite reach his Shabbos boundary, falling 2 cubits short? Or whether a wall of humans may be constructed at all on Shabbos? New Mishna: one may leave his Shabbos boundary for a mitzvah purpose like saving a life, witnessing the new moon in order to testify before the Sanhedrin that the new month has begun, or to fight a battle. If he leaves his boundary, however, and then learns his service is no longer required, what should he do?