(**  Qualifiers and keyed predicates.  **)

Variant qualifier (q : nat) T := Qualifier of {pred T}.(**  Qualifiers and keyed predicates.  **)

Variant qualifier (q : nat) T := Qualifier of {pred T}.Coercion has_quality n T (q : qualifier n T) : {pred T} :=
  fun x => let: Qualifier _ p := q in p x.

Lemma qualifE n T p x : (x \in @Qualifier n T p) = p x.