[If] someone surrounds his neighbor on three sides, and he fences off the first, second, and third [sides], we do not obligate him. R' Yose says: If he stood up and fenced off the fourth [side], we devolve [the obligation for] all of it upon him. 4. [If] the wall of a courtyard fell, we obligate him to rebuild it to [a height of] four cubits. [Each is] presumed to have contrib-uted until [the other] brings proof that he did not contribute. From four cubits and upward, we do not obligate him. [If] he build another wall near it, even though he did not [yet] place a roof upon it, we [nevertheless] devolve [the obligation for] all of it upon him. [He is] presumed not to have contributed until he brings proof that he did contribute.