(define (download source #!optional destination latest-rev progress-callback)
  (let ((proc (lambda ()
		(parameterize ((callback progress-callback))
			      (files:get (alist-ref 'root source)
					 (alist-ref 'path-key source)
					 rev: (if latest-rev #f (alist-ref 'rev source)))))))
    (old-output-port (current-output-port))
    (cond
      ((port? destination)
       (with-output-to-port destination proc))
      ((string? destination)
       (with-output-to-file destination proc))
      (else (proc)))))