do 4 intro.
assert (b <= i) by omega.
assert (i < b + S (length l)). omega.
cset (H i H2 H3).
unfold IndexIn in *.
destruct (In_map_inv H4). clear H4.
destruct H5.
subst i.
apply in_map.
inversion_clear H5...
subst.
elimtype False.
apply (le_Sn_n x)...
Qed.

Lemma vec_IndexSeq_nats_perm b n (v: Vector.t Index n): IndexSeq b v ->
vec.Permutation (vec.map nb_val v) (vec.nb_nats b n).