repeat intro.
apply lt_asym with (nb_val x) (nb_val y)...
apply IndicesCorrect.
apply Ecmp_apply_sym...
Qed.

(* Specifications: *)
Lemma IndexSeq_nil b: IndexSeq b nil.
Definition IndexSeq_uncons l b:
IndexSeq b l -> l <> nil -> exists l', exists p: Index, nb_val p = b /\ Permutation.Permutation (p :: l') l.