Inheritance of parameterized typed classes


Suppose I have the following typed class:

(define parent%
  (class object%
    #:forall (a)

    (: show-me (-> a Void))
    (define/public (show-me x) (displayln x))))

What is the best way to say that a class with the type variable a is the subclass of (inst parent% a)?

The following complains that a is unbound:

(define child%
  (class (inst parent% a)
    #:forall (a)

The following typechecks:

(define child%
  (class (inst parent%)

but since (inst parent%) is the same thing as (inst parent% Any), it's not exactly what I am looking for. I think I can live with it for now, but I wonder whether there is a better way to achieve parameterized inheritance.