Definition Index := natBelow (length ol).Definition subscript: Index -> T := vec.nth (vec.insertion_sort (@Ele T) (@Ele_le_dec T) ol).Definition UE: E :=
    Build_E (fun x y: Index => Ecmp T (subscript x) (subscript y))
      (fun x y => Ecmp_sym T (subscript x) (subscript y))
      (fun x y z => Ecmp_trans T (subscript x) (subscript y) (subscript z))
      (fun x y z => Ecmp_eq_trans_l T (subscript x) (subscript y) (subscript z)).Definition IndexIn (i: nat) (l: list Index): Prop := In i (map nb_val l).Definition IndexSeq (b: nat) (l: list Index): Prop :=
    forall i, b <= i -> i < b + length l -> IndexIn i l.Definition IndexSeq_above (b: nat) (l: list Index): Prop := IndexSeq (b - length l) l.

Lemma IndicesCorrect_inv (x y: Index): x < y -> @Ele UE x y.