here the 3dm "bugs" http://www.zbrush.fr/phpBB2//download.php?id=1055
architecture windows persistant (the rectangular block for closed the window by Boolean union) a rectangle persist!


and stairs (if you take the volumes to add, depending of their order, persistant lines are not the same!!

Ps like the volume "the wall + windows" above, does exist a reasonable limit of the numbers of windows(objects)
to booleanize ?(difference) (one by one or all in a same pass)