Didier Kryn writes:

>     Thanks to you all! I eventually discovered that my <delete> 
> key had also a tiny label saying sys-rq !
> Will try it tonight...

You can always get a daskeyboard or happy hacker and escape the tyranny of
built-in documentation. This is mine:
http://www.getdigital.de/images/actionshots/t4/dasKeyboardUltimateEU.jpg

Arnt