Catch shift key down

Hi!

Is it possible to catch a key down of the shift (or alt/option or  
control) key via CAPI? It doesn't seem to generate a :key :press event.

I work on Cocoa, and there is a event flagsChanged: on NSResponder. If  
there is not a clean CAPI solution, is it possible to catch this event  
in a Cocoa-specific solution?

Thanks
Erik