[plt-scheme] Units And Contracts In Bed(ed) Together

Is there a way to combine contracts with unit sigs?

;; Amazon S3 API
(define-signature s3^
  (list-buckets        (-> aws-credentials? any))
  (create-bucket       (-> aws-credentials? s3-bucket? any))
  (delete-bucket       (-> aws-credentials? s3-bucket? any))
  (put-object          (-> aws-credentials? bytes? s3-resource? any))
  (put-file-object     (-> aws-credentials? path? s3-resource? any))
  (head-object         (-> aws-credentials? s3-resource? any))
  (get-object          (-> aws-credentials? s3-resource? bytes?))
  (delete-object       (-> aws-credentials? s3-resource? any))
  (list-bucket-objects (-> aws-credentials? s3-bucket? s3-key? string?
number? any)))

Then given a unit

;; my naive s3 implementation
(define s3@
  (unit 
    (import ...^)
    (export s3^)

    (define list-buckets ...)
    (define create-bucket ...)))

The idea would be to attach the signature providing contracts to the
implementing procedures defined in the unit when the unit is invoked.