1) WALKING OVER EXCREMENT
QUESTION: Rav Ashi states that excrement lying in Reshus ha'Rabim is not considered a Makom Petur, but it is part of Reshus ha'Rabim.
Generally, areas where people do not walk are considered to be a Makom Petur and not Reshus ha'Rabim. Why, then, is excrement considered to be Reshus ha'Rabim if people do not step in it?
ANSWER: The MORDECHAI (#377) says that Rav Ashi is consistent with his opinion later (100b). Rav Ashi maintains that a narrow puddle less than four Tefachim wide is considered a Reshus ha'Rabim even though people do not step into it, since they step over it. Here, too, Rav Ashi says that since people walk over the excrement, that is enough to make it into Reshus ha'Rabim. (The OR SAME'ACH, Hilchos Shabbos 14:24, and SEFAS EMES to 100b suggest the same approach without mentioning the Mordechai.)
2) AN OPEN VALLEY
QUESTION: RASHI (DH d'Ad Asarah) implies that a Karpaf (open valley) which is larger than a Beis Se'asayim and was fenced-in but not for residential purposes is considered a full-fledged Karmelis. This seems to contradict the Gemara earlier which says that it is a Reshus ha'Yachid with the stringencies of a Karmelis. Rashi later (80a, 99b) reiterates his opinion that it is a full-fledged Karmelis and not a Reshus ha'Yachid. How are we to reconcile the words of Rashi with the Gemara? (GILYON HA'SHAS)
ANSWER: The Acharonim explain that RASHI (DH Kegon) stresses that a Karpaf that was not fenced-in for residential occupation is a Reshus ha'Yachid only when it did not have any houses in it before the wall was built around it. This implies that a Karpaf has the status of a Reshus ha'Yachid when, after the wall was built, a house was erected inside the Karpaf. Perhaps Rashi holds that a fenced-in Karpaf is considered a Reshus ha'Yachid only when a house was later built inside the walls. If no house was ever built inside the walls, the Karpaf remains a full-fledged Karmelis.