Sidepost visible from outside the alley but not inside. Proof from Rav Chiya that it works. Question from case of small courtyard open to large courtyard, ostensibly showing that it does not work. Walls of small courtyard extend into large courtyard and are not centered. Why don’t we say “lavud” and connect small cy wall to large cy wall and use that? Following opinion of Rebbe that requires 2 sideposts. Gemara proves conclusively from measurements of the two courtyards that visible from outside but not inside does not work.
Rav Yosef forgot his learning after an illness. Abaye retaught him everything he had learned from him. Gemara finishes off with a ruling that a sidepost visible from the outside but not inside does work, because Rav Chiya quoted such a Beraita, and he was very careful in his learning and reviewed every 30 days.
Opening wider than 10 according to Rav Yehuda, when to narrow it? Inconclusive
Wells in public domain are given certain leniencies to allow water access: larger open space and open space overwhelming closed space.
How to close an open wall of 20 cubits? Put a panel 10 handbreadths high by 4 cubits long in the middle of the space, or put up posts alternating with open space, where the posts are larger than the open space, umtil down to 10 cubits open. Gemara gives simplest option so as not to trouble people.
Case of toilet seat transferring ritual impurity to/from objects underneath.