Definition set := list A.Definition empty_set : set := nil.Fixpoint set_add (a:A) (x:set) : set :=
    match x with
    | nil => a :: nil
    | a1 :: x1 =>
        match Aeq_dec a a1 with
        | left _ => a1 :: x1
        | right _ => a1 :: set_add a x1
        end
    end.Fixpoint set_mem (a:A) (x:set) : bool :=
    match x with
    | nil => false
    | a1 :: x1 =>
        match Aeq_dec a a1 with
        | left _ => true
        | right _ => set_mem a x1
        end
    end.(** If [a] belongs to [x], removes [a] from [x]. If not, does nothing.
      Invariant: any element should occur at most once in [x], see for
      instance [set_add]. We hence remove here only the first occurrence
      of [a] in [x]. *)

  Fixpoint set_remove (a:A) (x:set) : set :=
    match x with
    | nil => empty_set
    | a1 :: x1 =>
        match Aeq_dec a a1 with
        | left _ => x1
        | right _ => a1 :: set_remove a x1
        end
    end.Fixpoint set_inter (x:set) : set -> set :=
    match x with
    | nil => fun y => nil
    | a1 :: x1 =>
        fun y =>
          if set_mem a1 y then a1 :: set_inter x1 y else set_inter x1 y
    end.Fixpoint set_union (x y:set) : set :=
    match y with
    | nil => x
    | a1 :: y1 => set_add a1 (set_union x y1)
    end.(** returns the set of all els of [x] that does not belong to [y] *)
  Fixpoint set_diff (x y:set) : set :=
    match x with
    | nil => nil
    | a1 :: x1 =>
        if set_mem a1 y then set_diff x1 y else set_add a1 (set_diff x1 y)
    end.Definition set_In : A -> set -> Prop := In (A:=A).

Lemma set_union_intro2 : forall (a:A) (x y:set), set_In a y -> set_In a (set_union x y).