(** 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)).(** 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)).(** 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)).

Lemma is_set_insertion_in : forall (A:Set)(l l':list A)(x:A),(insertion A x l l')->(is_set A l')->~(In x l).