Thank you Michael, I think I'm understanding more and more.

Your reply http://moi3d.com/forum/lmessages.php?webtag=MOI&msg=5461.10 helped me a lot understanding booleandiffrence. But if I run this script it looks fine at fist. But when I click on the resulting cylinder and move it, suddenly the result from the first booleandiffrence apears again.

Why is that?