Daf Yomi Moed Katan 18b: An Interesting Kelal Horaah: Shema Mina Telas - What is the reach and provenance of this statement?

Today, in daf Yomi, we read Moed Katan daf 18. I had a question the other day on this very daf, and this very sugya, in that it appeared to recommend biting toenails!

There is another interesting point here for those fascinated by kelalei horaah. In Rif's discussion of the daf, he notes that someone bases a ruling -- that even on chol haMoed, one must bite the nails rather than use a nail scissors/clippers -- on something derived from an incident of Rabbi Yochanan biting his fingernails in a synagogue {in our gemara, it is a Bet haMidrash, while Rif has a Be Kenishta, a synagogue}. Rif rejects this since the gemara explicitly derives three points of law from this incident, and what is suggested in not amongst the three. Rif follows:

{Moed Katan 18a}
ת"ר כשם שאמרו אסור לגלח במועד כך אמרו אסור ליטול צפרניו במועד דברי רבי יהודה
ורבי יוסי מתיר
וכשם שאמרו אסור לגלח בימי אבלו כך אמרו אסור ליטול צפרניו בימי אבלו דברי רבי יהודה
ורבי יוסי מתיר

אמר עולא הלכתא כר' יהודה באבל והלכתא כר' יוסי במועד
ושמואל אמר הלכתא כרבי יוסי בזה ובזה

דאמר שמואל הלכתא כדברי המיקל באבל וקי"ל כשמואל ול"ש צפרניו דיד ול"ש צפרניו דרגל כולהו שרו
מיהו ה"מ בשיניו אבל בגנוסתרי אסור דאמר ר' חייא בר אשי (א"ר אשי) א"ר ובגנוסתרי אסור

The Sages learnt {in a brayta}: Just as they said that it is forbidden to shave on chol haMoed, so they said that it is forbidden to cut his nails on chol haMoed. These are the words of Rabbi Yehuda. And Rabbi Yossi permits.
And just as they said that it is forbidden to shave during his days of mourning, so did they say that it is forbidden to cut his nails during his days of mourning. These are the words of Rabbi Yehuda. And Rabbi Yossi permits.

Ulla said: The halacha is like Rabbi Yehuda as regards mourning and like Rabbi Yossi as regards chol haMoed.
And Shmuel said: The halacha is like Rabbi Yossi by both this and that.

For Shmuel said: The halacha is like the words of the lenient one by mourning. And we establish like Shmuel.

And it matters not whether they are fingernails or toenails, all they permitted.
However, these words are with his teeth. But with a nail scissors, it is forbidden. For Rabbi Chiyya bar Ashi (cited Rav Ashi) cited Rav: And with a nail scissors it is forbidden.

וחזינן למקצת רבוותא דאמרי דוקא בתוך שלשים אבל בתוך שבעה אסור
ואנן לא ס"ל הכי אלא אפי' בתוך שבעה שרי
וה"מ בימי אבלו אבל במועד אפי' בגנוסתרי מותר
ואיכא מאן דאמר דבמועד נמי בגנוסתרי אסור ומייתי ראיה מיהא דאמר רב שימי בר אבא הוה קאימנא קמיה דר' יוחנן בבי כנישתא בחולו דמועדא ושקלינהו לטופריה בשיניה וקאמר מדשלקינהו בשיניה ש"מ דבגנוסתרי אסור
ואנן לא סבירא לן הכי דאי ס"ד דבגנוסתרי אסור אדאמר ש"מ תלת הוה למימר ש"מ ארבע ונמני ליה בהדייהו ומדלא אמר הכי ש"מ דבגנוסתרי מותר והאי דשקלינהו בשיניה משום דהוה בבי כנישתא דליכא גנוסתרי ולהכי לא חשיב לה בהדייהו ולהכי אצטריך למימר דהוה קאי בבי כנישתא משום דלא תגמר מיניה דבגנוסתרי אסור הלכך לא גמרינן מיניה אלא הנהו תלת בלחוד דגמרי רבנן ותו לא מידי:
And we have seen that a few {post-Talmudic} Sages say that specifically within 30 days of his mourning, but within 7 is it forbidden. And we do not hold this, but rather even within 7 is permitted.
And these words are regarding his days of mourning, but during chol haMoed, even with nail scissors is permitted.
And there is one who says that on chol haMoed as well, with nail scissors it is forbidden. And he brings a proof from this that Rav Shimi bar Abba said: "I was standing before Rabbi Yochanan in the synagogue on chol haMoed, and he cut his nails with his teeth," and he {this man deAmar} states that from the fact that he cut them with his teeth, we may derive that with a nail scissors it is forbidden.
And we do not hold so, for if you think that with nail scissors are forbidden, where it said "we deduce from this three things," it should have said "we deduce from this four things," and listed this {not to use nail scissors} among them. And from the fact that it does not say so, we deduce that with nail scissors are permitted. And this that he {Rabbi Yochanan} cut them with his teeth, this was because he was in the synagogue, where there are no scissors, and therefore it is not counted among them, and therefore it was necessary to state that he was in the synagogue, so that you do not deduce from it that with a nail scissors it is forbidden. Therefore we only learn from it those three that the Sages learned, and further nothing.

As summarized in the Point by Point Summary of Kollel Iyun haDaf, the three things derives are:

1. He held it is permitted to cut nails on Chol ha'Mo'ed
2. It is not gross to bite your nails in front of other people (Rashi MS).
3. One may throw one's nails on the floor.

Note that the second point is Rashi's explanation of what merely states that it is not mius. Biting nails specifically is not mentioned.

The incident, by the way, was that Rav Shemen bar Abba stood before Rabbi Yochanan in the study hall {or synagogue} and saw his remove his nails, and it does not specify with what, and thus does not mention this nail clippers. But this is not necessarily so. Rif's girsa is apparently equal to the person that he cites, where the gemara explicitly states that he removed his nails with his teeth. This difference in girsaot is also something that can influence how one will paskin, though not for Rif -- he does not need this to reject this. It is unclear what girsa Rashi had, where Rashi explains the mius as being because he removed it with his teeth.

Can we indeed make sure a derivation from an incident, even where the gemara gives a series of things derived, and a number of things derived? That is, did it really intend a closed class? Or was it just listing three things which one could derive from this?

Another interesting point is that even Rif does not explicitly say that it was Rav Shemen bar Abba who deduced these three points, just the "Rabbanan" who derived this. In other words, it is the gemara who deduces this.

Would we make the same judgment? Someone should do a study -- perhaps it has already been done -- on the construction of Shema Mina Telas, to try to determine its provenance. That is, do other Amoraim react to such derivations? If so, it would be early, and part of an early formulation. On the other hand, if it is stamaic, then perhaps it should not bear so much influence in determining halacha, such that we would exclude other derivations from it.