Definition fashionM {A} `{ageable A} : modality
  := exist _ fashionR valid_rel_fashion.Program Definition fash {A: Type} `{NA: ageable A} (P: pred A): pred nat :=
      fun n => forall y, n >= level y -> P y.Notation "'!' e" := (unfash e) (at level 30, right associativity): pred.

Lemma laterR_nat: forall (n n': nat), laterR n n' <-> (n > n')%nat.