Program Definition fstn_stream {A: Type} (n: nat) (h: stream A) : stream A :=
  fun m => if le_lt_dec n m then None else h m.Definition skipn_stream {A: Type} (n: nat) (h: stream A) : stream A.

Lemma skipn_stream_spec {A: Type}: forall n m (h: stream A), (skipn_stream n h) m = h (m + n).