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(define link-urls (make-hash))
;; Provided tag functions:
(define (html-link . args)
  `(link& [[ref ,(format "~a" (first args))]] ,@(rest args)))
(define (html-url ref url)
  (hash-set! link-urls (format "~a" ref) url))
(hash-set! link-urls (format "~a" ref) url) "")
;; Private use (by html-root):
(define (decode-link-urls tx)
  (cond [(eq? (get-tag tx) 'link&)
         (let* ([url-ref (attr-ref tx 'ref)]
                [url (or (hash-ref link-urls url-ref #f)
                         (format "Missing reference: ~a" url-ref))]) |