Axiom pl : (nat -> Prop) -> (nat -> Prop) -> (nat -> Prop).
Axiom plImp : forall k P Q,
pl P Q k -> forall (P':nat -> Prop),
(forall k', P k' -> P' k') -> forall (Q':nat -> Prop),
(forall k', Q k' -> Q' k') ->
pl P' Q' k.