-keyUp: with 'Shift', 'Ctrl' and 'Alt'

I try to receive the NSKeyUp event with only 'Shift', 'Ctrl' or 'Alt',
not the usual combination with alphabet.
I track down that gnustep-back indeed receive this kind of event from Xwindow,
but somehow gnustep-gui does not pass it through -sendEvent in NSApplication.
Is it intentional ?
If so, is there any easy way to work around ? like overrideing some methods ?

And 'Ctrl+TAB" seems not pass through, too.
But it may be the problem of my application.

Thanx a lot

Yen-Ju