how does the "deker na'utz" take away the biblical prohibition of charisha? (rashi on daf gimmel says that melachos on yom tov (before kneading) are biblically forbidden.
SHMUEL GOTTESMAN , BROOKLYN, NY
(a) I think you meant Tosfos 3a (DH Geneivah - see Rashi 23b DH Ein Tzadin and Tosfos there).
(b) It is not Choresh because the Deker makes the Charishah (i.e. Chafirah) take place before the start of Yom Tov (Rashi 7b DH v'Ha). Alternatively, Choresh doesn't apply here at all (as it would seem from Tosfos 8a DH v'Eino) because it is being done in a house, where there is no advantage in softening the remaining earth. Choresh only applies in a field (see Shabbos 73b).