inversion H;inversion H0;apply is_permutation_refl.
inversion H1.
apply cons_is_permutation with l'';trivial.
apply cons_is_permutation with l';trivial.
elim (insertion_trans A l' l'' x H0 l3 x0 H2).
intro l4;intro.
elim H8;intros.
apply cons_is_permutation with l4;trivial.
eapply IHis_permutation;eauto.
Qed.

(* Specifications: *)
(** is_permutation is symmetric *)
Lemma is_permutation_sym : forall (A:Set)(l l':list A),(is_permutation A l l')->(is_permutation A l' l).
induction 1;[apply nil_is_permutation | eapply permutation_insertion;eauto;apply head_insertion].