CoFixpoint zip (sA: infseq A) (sB: infseq B) : infseq (A*B) := 
  match sA, sB with
    | Cons a sA0, Cons b sB0 => Cons (a, b) (zip sA0 sB0)
  end.

Lemma always_map_conv : forall (f: A->B) (P: infseq A->Prop) (Q: infseq B->Prop), (forall s, Q (map f s) -> P s) -> forall (s: infseq A), always Q (map f s) -> always P s.