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_empty {A: Type}: forall n (h: stream A), is_at_most_n_stream n h -> skipn_stream n h = empty_stream.