case (factorisation n);intros;try tauto.
Qed.

(* Specifications: *)
(** if l and l' are two factorisation of n, then l' is a permutation of l *)
Theorem factorisation_unique_upto_permutation : forall (n:nat)(l l':list (nat*nat)),(is_factorisation n l)->(is_factorisation n l')->(is_permutation (nat*nat) l l').
unfold is_factorisation;intros.