(Mishnah): A Neder takes effect on another Neder, but a Shevu'ah does not take effect on a Shevu'ah;

How is this? If one said 'I will be a Nazir if I eat, I will be a Nazir if I eat', and he ate, he is obligated [to conduct one Nezirus] for each one (each time he said so);

If he said 'Shevu'ah, I will not eat. Shevu'ah, I will not eat' and he ate, he is liable only once.

(Gemara) Question (R. Yosi): 'Shevu'ah, Shevu'ah, Shevu'ah I will not eat' and he ate [what is the law? Granted, a Shevu'ah does not take effect on a Shevu'ah, but perhaps Shevu'os can take effect at once! - PANIM ME'IROS]

Answer (R. Yosi bei R. Bun): We learn from the following. 'Shevu'ah I will not eat this loaf, Shevu'ah I will not eat, Shevu'ah I will not eat', and he ate, he is liable only once, because he said 'this'.

Inference: Had he not said 'this', he would be liable for each one.

Avimi, Chaifah's brother: I learned Nedarim from my Rebbi, and I learned Shevu'os from my Rebbi.

Question: Chaifah wanted to test him. [He asked] if there were five loaves in front of him, and he said "Shevu'ah I will not eat this loaf", and returned and said '[Shevu'ah I will not eat] these two', and then said 'these three', and then said 'these four', and then said 'these five', and he ate the first, what is the law?

Answer #1 (Avimi): He is liable for each [Shevu'ah].

Rebuttal (and Answer #2 - Chaifa): He is liable only once. Once he said [about the first] "Shevu'ah", he made it like Neveilah. Afterwards, he is like one who puts a Shevu'ah on Isurim. Shevu'os do not take effect on Isurim.

Question: Chaifah tested him again. [He asked] if there were five loaves in front of him, and he said "Shevu'ah [I will not eat] these five loaves", and returned and said '[Shevu'ah I will not eat] four', and then said 'three', and then said 'two', and then said 'one', and he ate all of them [what is the law]?

Answer #1 (Avimi): He is liable only once.

Rebuttal (and Answer #2 - Chaifa): He is liable for each one. If one said "Shevu'ah I will not eat five loaves", and he ate four, is he not exempt?! (The Shevu'ah about four forbids something permitted until now, and likewise each subsequent Shevu'ah.)

R. Yosi: Presumably, Chaifa's brother is correct in the latter question (the first Shevu'ah also forbade each loaf itself), and Chaifa is correct in the first question.