[xmonad] How set new floating window gets on top ?

Hi, even with manageHook set to manageHook of defaultConfig, all new floating windows created on workspace with simplestFloat or simpleFloat layouts recieve focus but are placed _bellow_ current windows which I find very frustrating. For example on empty simplestFloat workspace I launch xterm, then launch gvim. Gvim gets focus but appears behind xterm (bellow in window stack sense). Maybe I'm not smart enough but I can't find how to change this behaviour of floating layout. I'd simply need all new floating windows are placed on top of the stack. Any idea ? Thanks in an advance. David