Convert strings to appropriate symbols

[Heime]

How can I ensure that the text strings from completing-read
are properly transformed to the symbols required by 
outline-minor-mode-highlight ?

(defun outl-flock (actm)
  "Set font-lock rules for 'outline-minor-mode'.
  (interactive
   (let ( (cseq '("append" "t" "nil" "override")) )
     (list
       (completing-read "Font-Lock Rules: " cseq nil t "override"))))

  (setq outline-minor-mode-highlight
         (if (equal actm "t")
             t
           (intern actm)))  )