eapply cons_is_permutation.
apply is_permutation_refl.
apply insertion_snoc.
Qed.

(* Specifications: *)
Lemma insertion_append : forall (A:Set)(x:A)(xs xss:list A),(insertion A x xs xss)->forall (yss:list A),(insertion A x (app xs yss) (app xss yss)).
induction 1.