Why some links are nofollow

gistfile1.arc

(= follow-threshold* 5)

; ...

(def titlelink (s url user)
  (tag (td class 'title)
    (if (cansee user s)
        (do (let toself (blank url)
              (tag (a href (if toself (item-url s) url)
                      rel  (unless (or toself 
                                       (> (realscore s) follow-threshold*))
                             'nofollow)) 
                (pr (s 'title))))
            (deadmark s user)
            (awhen (and (valid-url url) (sitename url))
              (spanclass comhead
                (pr " (" it ") "))))
        (pr (pseudo-text s)))))