Arguments eq_dec _ _ _ _ : simpl never.

Lemma upd_Znth_eq : forall {A} {EqDec : EqDec A} {d: Inhabitant A} (x : A) (l : list A) i, 0 <= i < Zlength l ->
upd_Znth i l x = map (fun j => if eq_dec j i then x else Znth j l) (upto (length l)).