Defaults for optional arguments for use with interactive

[Heime]

With the following function, how can I default actm to be 'list
so that the "(interactive (list" part guts executed rather than 
the "(interactive string" part ?


(defun xiakos-context (search-text &optional n actm)

  (setq n (or n 8))
  (setq actm (or actm 'list))

  (interactive

   (cond
     ((eq actm 'list)

         (list
           (read-string "Search Text: ")
           (read-number "Number of Context Lines: ")
           ('list)))

     (t

         "s - Search Text: \n\
n - Number of Context Lines: ") ))