031: The ashes of a stove are always considered muchan.Rav Yehuda: One can bring a basket full of dirt in, and use it for all his needs.Challenge: Beraisa: A koy should not be shechted on Yom Tov. If it was shechted, it may not be covered with earth.If what we learned is true, why can’t kisui be done in accordance with Rav Yehuda’s teaching?Resolution: Perhaps he doesn’t have any prepared dirt.
Challenge: The beraisa further states “If it was shechted, it may not be covered with earth”, this implies that earth is available?
Rabbah: Stove ashes are considered muchan only for needs that are certain, not for something that is only needed in case of a safek [such as for covering the blood of a koy].
Challenge: Why would it be forbidden in case of a safek? Because he is forming a hollow? A hollow is formed whenever the earth is removed, regardless of whether it will be used for safek! You must answer that a hollow is not an issue since we rule like Rabbi Abba (031), so why should it be problematic when doing it for a safek need?
Response: It is forbidden lest he come to grind the dirt.
Challenge: It should be just as forbidden in the case of a different animal?
Resolution: In that case the mitzah of kisui overrules the lav of grinding on Yom Tov [eseh docheh lo saaseh].
Challenge: Eseh docheh lo saaseh should not apply here. It only applies when both mitzvos are simultaneous: such as the case of milah on a tzaraas [which may not be removed] or shaatnez in tzitzis. But here when he is chopping up the block of earth – transgressing the melacha of grinding, he is not yet doing the mitzvah of kisui.
Resolution: If he’s grinding it over the blood, it is simultaneous.Challenge: In any event Yom Tov is not only a lav, it is also an asei. An eseh cannot override another eseh. 
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