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1. All agree that one may not build a new structure, even a temporary one, on Shabbos or Yom Tov.
2. Walls may be formed by making a human partition consisting of groups of people, so that a public domain is transformed into a Reshus ha'Yachid to permit carrying.
3. While partitions of people are effective, the people who form the walls may not be aware that they are serving this purpose.
4. A person is permitted to have people form a partition if they will not know why they are there.
5. A person who received permission to leave the Techum, and then was told that it was no longer necessary for him to leave the Techum, is allowed to walk 2,000 Amos from the place where he was told he could go.
A BIT MORE
1. Rebbi Eliezer: One is not even allowed to add to a temporary structure. Chachamim: One is permitted to add to a temporary structure.
2. If people are standing close together in a way that encloses an area of the public domain, it is considered a private domain and one may carry inside the area created.
3. The Gemara relates that Shmuel punished people who told others to stand in a certain area in order to make a human partition. Similarly, while Rava used this leniency to enable his assistant to carry within the throngs of people accompanying him on the way back from his lecture, Rava did not allow his assistant to do so the following week, reasoning that the people were already aware they were being used as a partition.
4. This is why Rav Chisda arranged for people to be in a certain area and form a private domain. He himself was not part of the wall.
5. For example, if a midwife was told that she was needed outside the Techum and then was told -- when she was already outside the Techum -- that she was no longer needed, she may walk 2,000 Amos in any direction from where she was told to go.
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