Rif Eruvin 11a {Eruvin 41b continues ... 42b}

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11a

{Eruvin 41a continues}
and returned deliberately, he would only have four cubits.

They inquired of Rabba: If one needed to use the bathroom {when restricted to a four cubit area}?
He said to them: Great is the honor of man, which supersedes even a biblical negative commandment.
The Nehardeans said: If he is smart, he will go into the techum {in order to answer nature's call} and once he has entered, he has entered.
Rav [Papa] said: Fruits which were carried outside of the techum and then returned, even intentionally, they do not lose their {initial} location. What is the reason? They were under duress {that is, they did not go in or out of their own accord}.

{Eruvin 42a}
Rav Nachman cited Shmuel: If he was travelling on the road and does not know techum Shabbat, he should take 2000 middle-sized steps, and this is techum Shabbat.
And Rav Nachman cited Shmuel: If one took his Shabbat-rest in a valley and gentiles erected a partition on Shabbat, he may walk 2000 cubits {inside the partitions}, and may move things throughout {even outside 2000 cubits} via throwing.
And Rav Huna said: He may walk 2000 cubits and may move things within 4 cubits, but throughout via throwing, no, lest he is drawn after his article {if he throws it past 2000 cubits}.
But let him at least carry normally within the 2000 cubits?!
It is like a partition which is breached in its entirety to a place forbidden to it.

{Eruvin 42b}
There is a brayta in accordance with Shmuel: If he was walking and measuring {the techum} and his measure ended in the midst of the city, it is permitted to carry in the entire city, so long as he does not pass the techum with his feet {from the place he set as his Shabbat center outside the city}.
And with what does he carry? Is this not via throwing?

To explain: Since he may not step with his legs over the techum, which is half the city, how does he carry through the entire city if not via throwing?

"If they brought him to another city...":
Rav said: The halacha is like Rabban Gamliel regarding a pen, a fold, and a boat.
And Shmuel said: The halacha is like Rabban Gamliel regarding a boat, but not a pen and a fold.

Regardless, all agree that the halacha is like Rabban Gamliel regarding a boat.