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 now_map_conv : forall (f: A->B) (P: B->Prop) (s: infseq A), now P (map f s) -> now (fun x => P (f x)) s.