Fixpoint filterMap {A B} (f : A -> option B) (l : list A) : list B :=
  match l with
    | [] => []
    | x :: xs => match f x with
                   | None => filterMap f xs
                   | Some y => y :: filterMap f xs
                 end
  end.

Lemma filterMap_In : forall A B (f : A -> option B) a b xs, f a = Some b -> In a xs -> In b (filterMap f xs).