The Mishnah (above, 2a, 1:b:1) permitted fixing problems with the water supply, specifically dredging out wells, etc. The implication is that only dredging is permitted, but not digging a new well.

(R. Yakov): This is only true if the public does not need the water (during the holiday - Ritva quoting Tosfos), but if the public does need the water (during the holiday) one may even dig a new well.

There are three Beraisa'os pertaining to this topic, and according to R. Yakov... the bottom line of them all is: If the water is needed (during the holiday) a private well may be dredged but not dug anew, and a public well may even be dug anew. If the water is not needed (during the holiday) a private well may be neither dredged nor dug anew, and a public well may be dredged but not dug anew.

R. Ashi infers from the Mishnah's words that R. Yakov is right. The Gemara tries to reject the proof, but in the end accepts it.

The Mishnah (above, 2a, 1:b:4) mentioned that graves are marked off on Chol ha'Mo'ed to prevent Tum'ah. The Gemara presents numerous allusions to the need to mark graves in the Torah or Nach.

Apropos to one of these verses, the Gemara talks about how important it is for a person to carefully consider the consequences of his actions before doing anything.

Marking is done only for Tum'ah that can be Metamei through Ohel (as opposed to Tum'ah that can be Metamei only through direct contact). For example, a k'Zayis of flesh is not marked.

Marking is not needed where the Tum'ah is well-known, but only where there is some uncertainty (Sechachos, Pera'os and Beis ha'Pras).

The marking should be not be placed directly over the Tum'ah, but at some distance (to give sufficient advance warning), but not too much distance (so as not to waste the land of Eretz Yisrael by making it Tamei).

The Beraisa (see 3:a) implies that a k'Zayis of flesh is not Metamei through Ohel.

Question: This contradicts a Mishnah, which states clearly that a k'Zayis is Metamei through Ohel.

Answer: The Beraisa refers to a piece of flesh which is exactly a k'Zayis, and will certainly decompose shortly to become less than a k'Zayis. It is better not to mark it, risking that someone will become Tamei from it in the short run, rather than to mark it and have people wrongly burn their Terumah and Kodshim because of it forever.

Definitions of the terms describing uncertain Tum'ah (in 3:b).

Sechachah - Tree branches, one of which leans over Tum'ah, but it is unknown which (Rashi).

Pera'os - Protrusions from walls, which may have Tum'ah under them. [3. Beis ha'Pras - A field containing a grave that was plowed over; the Tum'ah spreads 100 Amos in all directions. This interpretation is later rejected, and according to many Rishonim it should be erased from the text here.]

Question on Beis ha'Pras (B.H.): The Beraisa implies that B.H. is Metamei through Ohel. This is contradicted by what Shmuel said, that a person may bend down and blow his way through a B.H.

Answer: The word B.H. actually applies to three different kinds of field: A) A field where a grave was plowed over, as above. B) A field that contains a grave in it, but no one remembers exactly where. C) A "field of weeping" (a field used for bewailing the dead brought from faraway places, where a piece of the corpse may have fallen away). A is not Metamei through Ohel, and does not require marking. B is the one that requires marking, and it is Metamai through Ohel.

Question: A Beraisa talks about someone who finds a field marked off as a B.H. but doesn't know whether it's type A or type B. (The Beraisa determines that if there are trees it's A, if not it's B. R. Yehudah says its status remains unknown unless there is an old man or rabbinical student who knows for sure.) Hence, we see that both types of field are marked!

Answer: What the Beraisa means is that the field was definitely type B when it was marked, but a doubt exists as to whether it was plowed over subsequently (thus becoming a type A) and therefore no longer Metamei through Ohel.

Question: The Beraisa said (4:b:6) that if there are trees the field was definitely plowed (for the sake of the trees growing there). Perhaps only the area near the trees was plowed, but the area between the trees and the road is still Metamei through Ohel.

Answer: The Beraisa is talking about a case where the trees are next to the road.

Question: Perhaps the area beyond the trees was not plowed and is still Metamei through Ohel.

Answer #1: The Beraisa is talking about a case where the trees are scattered throughout the field.

Answer #2: The border of the marking is near the trees.

Observation: The Beraisa (above, 5b, 4:b:6) mentioned that according to R. Yehudah the status is considered unknown unless there is an old man or rabbinical scholar there. A rabbinical scholar is responsible for keeping track of matters of his community.