induction H;simpl in H0.
case H0;intro H1;try (symmetry in H1);tauto.
case H0;simpl;intro;tauto.
Qed.
(** a list is a set iff all the elements are pairwise distinct *)
Inductive is_set (A:Set) : list A->Prop :=
nil_is_set : (is_set A nil)
|cons_is_set : forall (x:A)(l:list A),(is_set A l)->~(In x l)->(is_set A (x::l)).

(** if (insertion x l l') and l' is a set then l is a set *)
Lemma is_set_insertion : forall (A:Set)(l l':list A)(x:A),(insertion A x l l')->(is_set A l')->(is_set A l).
induction 1;intros.