Rif Eruvin 12a {Eruvin 43b continues ... 45a}

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12a
{Eruvin 43b continues}
is dwelling in misery {being stuck outside the techum for the remainder of Shabbat}.
He said him: Construct a partition composed of people, for then it will be as if he were within a pen or fold, in which he can traverse the entirety, and thereby enter {the techum}.

And this is specifically if he left {the techum} unintentionally, but if he left deliberately, no.

{Eruvin 44b}
And those {people} who compose the partition must also do so without knowledge {that they are constructing a partition by gathering in this formation}, but a partition which is constructed deliberately is forbidden. And if you say that Rav Chisda constructed a partition deliberately, for Rav Nachman said to him: Construct a partition composed of people and he will enter {the techum} thereby -- Rav Chisda was not among those who were part of the partition {and those people did not know they were part of a partition}.

There were certain gardeners who brought in water via a partition composed of human beings, and Shmuel administered lashes to them. He said: They {the Sages} said this {about using a partition made of standing humans} without knowledge {of the participants}. Did they say this with knowledge?!

Some bottles made of skin were once lying about in the manor of Mechoza {which was a public domain} and when Rava came in from his discourse, he {Rashi: his assistant} carried them in. {Since there were many people with Rava, who thus formed a human partition.} The next Shabbat, they wished to do the same, and he forbade them, for it was like it {the construction of the human partition} was done deliberately and thus forbidden.

Mishna:
If a person went out with permission {to do some required deed}, and they said to him, "The deed has already been done" he has 2,000 amot in any direction; if he was within the bounds, it is as though he had not gone out, for all who go out to save may return to their place.

Gemara:
What is this ruling saying?
Rabba said: This is what it means to say: If he was {still} in his {initial} techum, it is as if he never left his house.
But this is obvious!?
I would have said that since he uprooted himself {to leave the techum} he is uprooted. Therefore it informs us {that it is as if he never left his house}.

Rav Shimi bar Chiyya said: This is what it means to say: If the {new} techum which the Rabbis established for him overlapped his original techum, it is as if he never left his original techum.

And in this do they {Rabba and Rav Shimi bar Chiyya} argue. One Master {Rabba} holds that overlapping techumin are not significant, and one Master {Rav Shimi bar Chiyya} holds that overlapping techumin are significant. And {Rav Shimi bar Chiyya holds this} even though he did not rest {shavat} in the space of partitions. {A reference to, e.g., a cave, even 4000 cubits, in which there is an idea that overlapping techumin are significant, even according to Rabba.}

{Eruvin 45a}
Abaye asked Rabba from this that we learn {tnan}, "Rabbi Eliezer says: {If he walked beyond the techum} two {cubits}, he may reenter, three, he may not reenter." Is Rabbi Eliezer not consistent with his reasoning, that he maintains that {in terms of the 4 cubits he may walk} he is in the center, and the four cubits that the Sages granted him are as if they overlap the techum. {Thus, 2 cubits in each direction, so that 2 cubits, they overlap, while three do not.} And he says {by 2 cubits} that he may enter. Thus it is clear that overlapping techumin are significant.

And the Sages only argue with Rabbi Eliezer by {a case where he left the techum for} a matter of reshut {i.e. there was no necessity that he leave the techum}, but {if he left} for a matter of mitzvah, the Sages would agree to him.

And Rabba does not give any answer. Therefore, the halacha is like Rav Shimi bar Chiyya who holds that overlapping techumin are significant.

To explain {leaving} with permission: with permission of Bet Din.

"for all who go out to save may return to their place":
We establish that this means that they return with their weapons to their place, for they learnt {tnan}: Initially, they would lay their weapons in a house close to the wall. One time the enemies recognized them {later in the day while they were outside of the town}