fact: Fixpoint update {A} (L : list A) (i : nat) (x : A) : list A := match L, i with | _ :: L', 0 => x :: L' | h :: L', S i' => h :: (update L' i' x) | nil, _ => L end.
imports: Psatz List Arith.Div2 Even Relations RelationClasses PeanoNat Peano_dec Recdef Sorting Permutation Multiset