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1. Rav Nachman explains how one should measure his Techum when he has no method of standard measurement.
2. The Gemara discusses a case in which a person established his residence in a valley on Shabbos. Thereafter, Nochrim surrounded it with walls.
3. Everyone agrees that in the above case (#2), one has only 2,000 Amos from where he established his place residence.
4. There is a dispute about whether one may carry within these walls (#2) for more than four Amos at a time.
5. The Gemara discusses the opinion of Chiya bar Rav.
A BIT MORE
1. He should measure 2,000 average-sized steps from the place he is residing on Shabbos. This is his Techum.
2. In this case, the Nochrim surrounded it with walls in order to live inside the area in the valley. Had they surrounded it for other reasons, the Jew clearly would not have been permitted to carry anything within the walls for more than four Amos at a time.
3. The law that a person's entire home is considered his residence for Shabbos (and thus he may walk anywhere within his home despite its size, and another 2,000 Amos outside of his home) applies only when that was his residence at the onset of Shabbos. In this case, the walls were not in existence at the onset of Shabbos, and thus his original Techum is not changed.
4. Rav Nachman: One may throw an object more than four Amos within his Techum. Since he is in a private domain, there is no concern that he will end up going out of his Techum when he wants to retrieve his object. Rav Huna: He may not carry more than four Amos, because there is a concern that he will end up going out of the Techum.
5. The Gemara initially assumes he is a third opinion, but then concludes that his is the same as Rav Huna's opinion.
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