(with-output-to-file "testfile.ss"
  (lambda ()
    (printf "(\n")
    (let loop ([i 1000])
      (unless (zero? i)
        (printf "x ")
        (loop (sub1 i))))
    (printf ")\n"))
  'truncate)

(define sfd
  (let* ([ip (open-file-input-port "testfile.ss")]
         [sfd (make-source-file-descriptor "testfile.ss" ip)])
    (close-port ip)
    sfd))

(parameterize (#;[current-make-source-object
                (lambda (sfd bfp efp)
                  (make-source-object sfd bfp efp))])
  (call-with-input-file "testfile.ss"
    (lambda (in)
      (time
       (let loop ([j 100000])
         (unless (zero? j)
           (file-position in 0)
           (get-datum/annotations in sfd 0)
           (loop (sub1 j))))))))