[STUMP] Dim inactive monitor in dual head configuration

Hello again.

I've configured Stumpwm to run on a dual head configuration. But I 
always forget which monitor is currently active and start typing in a 
wrong window. I found a Python script that dims the inactive monitor a 
bit (See 
http://askubuntu.com/questions/665155/how-to-highlight-current-screen-or-window). 
It runs in a loop, sleeping for a 0.5 second after every run. Feels very 
sluggish, so I thought I could write a Lisp function that would dim the 
inactive monitor every time I move focus to and fro.

Here's what I've come up with:

(ql:quickload "cl-utilities")
(defun parse-xrandr ()
   (loop for i in (split-string (run-shell-command "xrandr" t))
      when (search " connected" i)
      collect (car (cl-utilities:split-sequence #\Space i))))

(defun dim-inactive-head (arg1 arg2)
   (let* ((heads (parse-xrandr))
	 (brighthead (slot-value (current-head) 'number))
	 (dimhead (if (eql brighthead 0) 1 0)))
     (run-shell-command
      (format nil "/usr/bin/xrandr --output ~d --brightness 1.0" (nth 
brighthead heads)))
     (run-shell-command
      (format nil "/usr/bin/xrandr --output ~d --brightness 0.6" (nth 
dimhead heads)))))

(add-hook *focus-frame-hook* 'dim-inactive-head)
(add-hook *focus-group-hook* 'dim-inactive-head)

The function uses cl-utilities:split-sequence, hence ql:quickload call 
in the beginning. Is there an alternative that doesn't have any 
dependencies?

It kinda works. I'm a bit worried by the fact that on one occasion the 
call to xrandr got stuck and didn't return, effectively putting the 
whole Stumpwm to stop. Besides I'm not sure if the hook is triggered 
when window placement rules switch groups. It would be nice if someone 
helped with testing and especially with fixing this straightforward 
implementation :)

In the beginning I used a wrong hook, *focus-window-hook*, but for some 
reason it didn't let me open any new window. Every time I tried to open 
xterm or anything else, Stumpwm stuck up. Strange, I thought that tiny 
function might be called any time, any place. It shouldn't have any 
harmful side effects.