Eruvin, Chapter 3, Mishnah 5. This mishnah deals with the question of how a person can place an eruv - in fact, two possible eruvin - and only afterwards decide which of them will take effect.
"Masneh adam al eruvo":
A person may attach a condition to his eruv. The case is one in which he placed one eruv - say, an eruv techumin - a thousand amos to the east of his house, and a second eruv a thousand amos to the west of his house, and at the time he placed them he does not yet know which of them he will want to take effect.
Making a condition because of non-Jews:
"Im ba'u goyim min hamizrach - eruvi lama'arav" - if non-Jews from whom he wishes to distance himself come from the east, the eruv placed to the west takes effect, so that he will be able to move even further westward away from them.
"Im min hama'arav - eruvi lamizrach" - and if they come from the west, the eruv placed to the east takes effect.
"Im ba mikan umikan - limkom she'ertzeh eilech" - if they came from both directions, he may choose in which direction to distance himself, preferring the group from which he wishes to distance himself less.
"Lo ba lo mikan velo mikan - hareini kivnei iri" - and if they did not come at all, he does not want either eruv to take effect, but rather that his status be like that of the rest of the residents of his city.
Making a condition because of the arrival of a Torah scholar:
Similarly, one may make a condition on account of the arrival of a Torah scholar who is going to deliver a lecture, when the one making the condition does not know from which direction he will come (the scholar himself is able to come because he placed an eruv techumin from the other direction):
"Im ba chacham min hamizrach - eruvi lamizrach" - so that he will be able to go out toward him and hear his lecture.
"Min hama'arav - eruvi lama'arav".
"Ba lechan ulechan - limkom she'ertzeh eilech" - if two scholars came, the decision is his, and the eruv he chooses is the one that takes effect.
"Lo lechan velo lechan - hareini kivnei iri" - if neither of them appeared, no eruv takes effect, and his place of Shabbos rest is his home.
The dispute between Rabbi Yehudah and the Sages:
Rabbi Yehudah: "Im hayah echad mehen rabo - holech etzel rabo" - when one of the two scholars is his teacher, he must go to his teacher, since that is presumably his natural preference. But "Im hayu shneihem rabosav - limkom sheyirtzeh yelech" - if both of them are his teachers, the choice is his, and the eruv he chooses is the one that takes effect.
The Sages: they disagree and hold that even when one of them is his teacher this is not decisive, since at times a person prefers to hear a lecture from the other scholar, who is a greater Torah scholar, rather than from his own teacher.
The principle of bereirah:
In all of these scenarios we are speaking of a person who does not reveal where he wants the eruv to take effect until after Shabbos has already begun. Ordinarily we say that the time an eruv techumin takes effect is at the onset of Shabbos, at bein hashemashos. The mishnah therefore requires the principle of bereirah: a person can say something and have it apply retroactively.
Accordingly, when on Shabbos it becomes clear to him from which direction those people from whom he wishes to distance himself have come, or from where his teacher has come, he can decide at that point and say that he wants the eastern or the western eruv to have taken effect already from the beginning of Shabbos - and by force of bereirah, which is a retroactive determination, his decision takes effect.
In summary: in this mishnah we learned that a person may attach a condition to his eruv and place two eruvin, in two directions, stipulating in advance which of them will take effect - whether because of non-Jews from whom he wishes to distance himself or because of a scholar whose lecture he wishes to hear; and if the condition was not fulfilled at all, his status is like that of the residents of his city. We examined the dispute between Rabbi Yehudah and the Sages regarding his teacher, and the principle of bereirah, which allows the determination to apply retroactively, from the beginning of Shabbos.