(* The converse seems to require much more work *)

Definition fstAB := fst (A:=A) (B:=B).

Lemma inf_often_map_conv : forall (f: A->B) (P: infseq A->Prop) (Q: infseq B->Prop), extensional P -> extensional Q -> (forall s, Q (map f s) -> P s) -> forall (s: infseq A), inf_often Q (map f s) -> inf_often P s.