Fixpoint subseq {A} (xs ys : list A) : Prop :=
  match xs, ys with
    | [], _ => True
    | x :: xs', y :: ys' => (x = y /\ subseq xs' ys') \/ subseq xs ys'
    | _, _ => False
  end.

Lemma subseq_filterMap : forall (f : B -> option A) ys xs, subseq xs ys -> subseq (filterMap f xs) (filterMap f ys).