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1. The Gemara explains the dispute between the Tana Kama and Rebbi Eliezer in the case of an inaccessible Eruv (when the key to the cabinet containing the Eruv was lost; see Daf 34, #4).
2. Rebbi Shimon states that roofs, yards, and large spaces are all considered one domain for vessels that were in them when Shabbos entered.
3. The Mishnah discusses various cases in which the Eruv was rendered invalid after it was set in its place.
4. There is a dispute about an Eruv which is discovered to be invalid on Shabbos, and which may have already been invalid during twilight on Friday evening.
5. The Gemara qualifies the case of an Eruv that rolls outside the Techum.
A BIT MORE
1. Tana Kama: Even if the key is found somewhere in the city, the Eruv is invalid, since the key cannot be carried to the cabinet. Rebbi Eliezer: The Eruv is valid if the key is in the city, since one can technically reach the cabinet based on Rebbi Shimon's leniency (see #2 below).
2. Rebbi Shimon maintains that one may carry such vessels from one of these domains to the other. Since Rebbi Eliezer (#1) agrees, he rules that if the key is found in one of these places, it may be transported to another similar place -- where the cabinet is located -- and therefore the Eruv is valid.
3. For example, if the Eruv was blown out of the Techum, was burned, or if it was Terumah that became Tamei (and any of these things occurred before twilight on Friday evening), the Eruv is invalid.
4. Rebbi Meir and Rebbi Yehudah: His Techum is determined by taking into account that either his house or his Eruv is his real Techum. He therefore may walk only within the 2000 Amos between his Eruv and his house. Rebbi Yosi and Rebbi Shimon: A doubtful Eruv is considered valid.
5. If it is less than four Amos outside the Techum, it is still valid.
Next Daf Index to Revach for Maseches Eruvin