Fixpoint partial_stream_clen {A: Type} (h: nat -> stream A) (n: nat): nat * nat :=
  match n with
  | 0 => (0, 0)
  | S n => let (k, m) := partial_stream_clen h n in
           match h k (S m) with
           | Some _ => (k, S m)
           | None => (S k, 0)
           end
  end.Fixpoint partial_stream_clen' {A: Type} (h: nat -> stream A) (n: nat): option (nat * nat) :=
  let (k, m) := partial_stream_clen h n in
  match n with
  | 0 => match h k m with
         | Some _ => Some (k, m)
         | None => None
         end
  | S n => match h k m, partial_stream_clen' h n with
           | Some _, Some _ => Some (k, m)
           | _, _ => None
           end
  end.

Lemma partial_stream_clen'_fin0_right {A: Type}: forall (h: nat -> stream A) (n m: nat), n > 0 -> is_n_stream n (h 0) -> partial_stream_clen' h (m + n) = match partial_stream_clen' (fun k => h (S k)) m with | Some (k1, k2) => Some (S k1, k2) | None => None end.