Gemara

(Mishnah - R. Yosi ben ha'Meshulam): One who slaughters a Bechor uproots hair to make room for Shechitah, using a large chopping knife.

(Rav Huna and Rav): It is permitted even on Yom Tov (because it is a Davar she'Eino Miskaven, i.e. he does not intend to uproot hair, only to clear it away).

Question: Rav forbids plugging up a barrel tightly on Yom Tov (even though he does not intend to squeeze out liquid)!

Answer: That is different. Even R. Shimon (who permits Davar she'Eino Miskaven) forbids that!

(Abaye and Rava): Even R. Shimon forbids a Pesik Reisha (something that will definitely result in a transgression. It is as if he intends to do it.)

Question: Rav rules like R. Yehudah (who forbids Davar she'Eino Miskaven).

Answer (and retraction): Rather, Rav permits on Yom Tov because it is an abnormal way of uprooting something from where it grows.

Shabbos 121b (Rav Yehudah): One may trample spit l'Fi Tumo (unaware of the consequences).

(Rav Sheshes): One may trample a snake l'Fi Tumo.

(Rav Katina): One may trample a scorpion l'Fi Tumo.

Shabbos 95a: Ameimar permitted Zilsa (to sprinkle water on the floor) in Mechuza. (All the houses had stone floors).

Ameimar: Chachamim forbade this lest one even out depressions (in a dirt floor). In this city there are no depressions.

140b (Mishnah - R. Dosa): One may sweep in front of an animal being fattened (so its food will not get mixed with dirt);

Chachamim forbid.

Rav Chisda: They argue about a feeding trough with a dirt floor. If it is a Keli, all permit sweeping.

Objection: No one would permit sweeping a dirt floor, for this evens out holes!

Correction (Rav Chisda): They argue about a Keli. If it is a dirt floor, all forbid.

Rishonim

Rif (48a): Nowadays that we hold like R. Shimon, a broom made from a date branch is a Keli she'Melachto l'Heter (one may use it on Shabbos)..

Rebuttal (Ba'al ha'Ma'or): Even R. Shimon forbids sweeping, for it is a Pesik Reishei that one will even out holes.

Defense (Milchamos Hash-m): The Rif has a tradition from Ge'onim that the Gemara means that nowadays that we hold like R. Shimon, even sweeping is permitted. Surely, one does not even out holes every time he sweeps, or even most times.

Rambam (Hilchos Shabbos 21:2): Anyone who evens out holes is liable for plowing. One may not spit on the ground and rub it with his foot lest he evens out holes. One may step on spit on the ground and walk l'Fi Tumo.

Magid Mishneh: See what I wrote in Perek 11.

Magid Mishneh (11:4): The Ramban and Rashba permit to intend to kill a snake or scorpion, just he makes it look like he does not intend, since we hold like R. Shimon, who exempts Melachah she'Einah Tzerichah l'Gufah (one does not benefit from it, or it is not for the same use as in the Mishkan). The Rambam rules unlike R. Shimon, but even so also he permits to intend.

Rambam (3): It is forbidden to sweep the ground unless it is tiled with stones.

Poskim

Shulchan Aruch (OC 316:10): In a place where snakes or scorpions do not kill, if they are not chasing a person, he may not (overtly) kill them, but he may step on them l'Fi Tumo, even intentionally, just he makes it look like he does not intend.

Shulchan Aruch (11): One may not rub, with his foot, spit on the ground, lest he even out holes. One may step on it l'Fi Tumo without intent to smear and even out holes.

Beis Yosef (DH v'Chen b'Rok): Rashi explains that he tramples it l'Fi Tumo, i.e. he does not intend to smear and even out holes. Even though automatically he smears, since he does not intend, it is permitted, since it is repulsive. An accurate text of the Tur says so. The Meforshim explain that even if he intends, but he does so l'Fi Tumo, i.e. he makes it appear as if he does not intend, it is permitted. Even though the Meforshim connote that we do not distinguish trampling l'Fi Tumo between spit and a snake or scorpion, it seems to me that for spit we permit only intent to trample, but not intent to smear. Even though for a snake or scorpion we permit even intent to kill, there is different, for they damage. Also, there the primary action itself is killing, but here the primary action is not smearing, for when he steps on it, this annuls the repugnance. (I.e. therefore there is no need to permit smearing - PF.) The Meforshim did not (see a need to) write this, for it was clear to them.

Beis Yosef (DH v'Chen Kasav): It seems that the Tur's text of the Rambam said 'we (the Gemara) said only that he tramples it.' Alternatively, it is his Perush why the Rambam wrote what he wrote.

Gra: The Shulchan Aruch is like Rashi. Even though the Ran wrote here like he wrote regarding a snake, and all the Meforshim connote like this, and Rashi holds like he wrote about a snake that one may not intend (to kill it), the Beis Yosef did not want to rule like the Meforshim (to be lenient, since) it is only due to repugnance.

Kaf ha'Chayim (101): The reason this Halachah is in this Siman is for contrast to the previous Halachah. Here, one may not intend to trample it.

Magen Avraham (24): The Shulchan Aruch connotes that it is forbidden on the ground, but on the floor it is permitted according to the opinion that permits sweeping a tiled floor (337:2, brought below). The Yerushalmi says so. All permit on a bench. This is difficult. We should forbid due to smearing! I answer that smearing applies only when he wants to smear one thing on another. Here, he wants the spit to be absorbed in the ground. Maharil says that one may stand on it, as long as he does not rub, and so says Hagahos Minhagim.

Mishnah Berurah (49): On the floor depends on the argument in 337:2. The Rema forbids there in every case, so he would say the same here.

Shulchan Aruch (ibid.): Even though he smears, since he does not intend, it is permitted, since it is repulsive.

Taz (10): It is a Pesik Reishei that he smear, and R. Shimon (normally) agrees that it is forbidden. Here it is permitted since it is repulsive.

Kaf ha'Chayim (107): Olas Shabbos and Tosfos Shabbos say that we must say that it is not a Pesik Reishei, for if so it is forbidden. However, the Tur connotes like the Taz, and Mishbetzos Zahav agrees with the Taz.

Mishnah Berurah (50): The Shulchan Aruch means that sometimes he smears, and we should be concerned lest he forget and intend and even out holes. Even so, it is permitted since it is repulsive.

Kaf ha'Chayim (106): The Drishah derives that one may smear as he walks. People who are meticulous not to, but they need not be. However, if he did not need to walk there, surely one may only step on it.

Magen Avraham (25): Nowadays that no one is adamant about the repugnance of spit, one must be careful not to rub when he steps on it. Perhaps this is what Maharil meant. However, fluid (phlegm and mucus) that comes out of the mouth or nose, since it is repulsive. In a Beis ha'Keneses, even spit is permitted. See 90:13 (brought below).

Kaf ha'Chayim (108): The Acharonim wrote like the Magen Avraham, so one should be stringent like them.

Mishnah Berurah (51): In a tiled house, one should not be more stringent about spit than phlegm.

Bi'ur Halachah (DH Shari): The Eliyahu Rabah argues with the Magen Avraham, and is equally lenient about spit and phlegm. Also the Drishah connotes like this. Even though the Acharonim wrote Stam like the Magen Avraham, regarding a tiled floor in any case some are lenient, so one may rely on them. Also this is like a decree for a decree, to be stringent about a tiled floor due to a dirt floor. The Isur regarding a dirt floor is lest one forget and intend to even out holes.

Mishnah Berurah (51): Even though when he tramples on the spit he often rubs it a bit, even so Chachamim permitted since it is repulsive. Since he does not rub it all around, it is not a Pesik Reishei. This is only as he walks, even if he intends to walk on it, but he may not intend to smear it. He may not walk to the place of the spit in order to trample it. This is not called l'Fi Tumo. Rather, he goes and steps on the spit, and he is careful not to rub at all. This is permitted in every case.

Kaf ha'Chayim (105): The Eshel Avraham says that the Shulchan Aruch connotes that smearing applies to spit, if he would want to smooth and spread it (e.g. on a wound). However, the Beis Yosef (328, regarding a bandage) says that smearing does not apply (even) to a very soft cream.

Shulchan Aruch (337:2): It is forbidden to sweep the floor unless it is tiled. Some permit even if it is not tiled.

Rema: Some forbid even if it is tiled. This is our custom, and one should not deviate.

Shulchan Aruch (90:13): One may spit in a Beis ha'Keneses. He tramples it with is foot, or covers it with a reed.

Kaf ha'Chayim (95): If one sees uncovered spit in the Beis ha'Keneses, it is a Mitzvah to cover it, due to the honor of the Beis ha'Keneses, and so others will not be repulsed.