(** [partition] *)

    Fixpoint partition (l:list A) : list A * list A :=
      match l with
      | [] => ([], [])
      | x :: tl => let (g,d) := partition tl in
                   if f x then (x::g,d) else (g,x::d)
      end.

Lemma filter_ext_in : forall (f g : A -> bool) (l : list A), (forall a, In a l -> f a = g a) -> filter f l = filter g l.