(Mishnah): If someone went even one Amah outside the Techum, he may not re-enter;

R. Eliezer says, if he went two Amos outside he may re-enter. If he went three, he may not enter.

If one was traveling and he was outside the Techum when it became dark, even if he was only one Amah outside he may not enter;

R. Shimon says, even if he was 15 Amos outside he may enter, for those who measure do not put the Techum marker at the full 2000 Amos, due to people who err [when measuring the Techum.]

(Gemara - R. Acha citing R. Chinena, and Rav Chisda): [R. Eliezer permits] only if he went two [Amos outside] - if he went two and Mashehu, he may not enter.

R. Eliezer teaches [like he taught elsewhere, that a person is in the middle of his four Amos. Therefore,] if he went two outside, he may enter. Three (or even two and Mashehu), he may not enter. (We explained this like SEFER NIR.)

Objection (R. Yosah): The Seifa [of R. Eliezer's words] says "three, he may not enter.'' This implies that three less Mashehu, he may enter!

Assumption: R. Eliezer discusses only one who left the Techum, but Machshich (he was outside the Techum when Shabbos came), no (he may not enter even if he is one Amah outside, for he was never inside the Techum).

Rejection: A Beraisa was found that says that this and this are the same. (In both cases he may enter if he is two Amos outside.)

Assumption: R. Shimon discusses only one who was Machshich [for this is Ones], but if he left the Techum, no (he does not allow 15 Amos).

Rejection: A Beraisa was found that says that this and this are the same.