Program Definition mkPred {A} `{ageable A} (p:A -> Prop) : pred A :=
  fun x => forall x', necR x x' -> p x'.(* The semantic notion of entailment.
 *)
Definition derives {A} `{ageable A} (P Q:pred A) := forall a:A, P a -> Q a.(* "valid" relations are those that commute with aging.  These
   relations are the ones that can be turned into modalities.
 *)
Definition valid_rel {A} `{ageable A} (R:relation A) : Prop :=
  commut A age R /\ commut A R age.(* A modaility is a valid relation *)
Definition modality {A} `{ageable A} := { R:relation A | valid_rel R }.(* More black magic to make the definition of modaility mostly opaque. *)
Definition app_mode {A} `{ageable A} (m:modality) : A -> A -> Prop := proj1_sig m.Definition mode_valid {A} `{ageable A} (m:modality) := proj2_sig m.

Lemma valid_rel_later {A} `{ageable A} : valid_rel laterR.