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Text Excerpt
ולא אמרן אלא דלא אקדימו הנך ואזמון עלייהו בדוכתייהו, אבל אזמון עלייהו בדוכתייהו — פרח זימון מינייהו.
We only said this halakha in a case where those members of the previous groups did not include them in the zimmun in their original place, but in a case where they included them in the zimmun in their original place, their obligation to participate in a zimmun has left them. The obligation incumbent upon these 3 individuals to form a zimmun stems from their obligation to form a zimmun with the members of their original groups. If their groups already included them in a zimmun, their obligation as individuals has lapsed and they can no longer form another zimmun.
אמר רבא: מנא אמינא לה — דתנן: מטה שנגנבה חציה, או שאבדה חציה, או שחלקוה אחין או שותפין — טהורה. החזירוה — מקבלת טומאה מכאן ולהבא.
In order to explain the general principle contained in this halakhic ruling, Rava said: From where do I derive to say this halakha? As we learned in a Mishnah: A ritually impure bed, half of which was stolen or half of which was lost, or it was divided by brothers after they inherited it from their father, or was divided by partners, it is ritually pure. This is true with regard to any ritually impure utensil that was broken or divided; it is no longer a utensil and is therefore ritually pure. However, if they restored it and reattached the parts, it is susceptible to ritual impurity from here on.
Rava infers: From here on, yes, it is susceptible to ritual impurity, retroactively, no, it does not reassume its previous status of ritual impurity. Apparently, once they divided it, the ritual impurity left it. Although it was restored, it does not reassume its previous status of ritual impurity. Here, too, once they included them in the zimmun, their obligation left them and they do not reassume their previous obligation.