widgets transparent for events

I have a group with an attached arm callback. In this group, there are a
couple of PtLabel widgets. Now, when I click on one of the labels, the arm
callback is not called. Now, I could attach the arm callback to all labels
as well, but I’d like to avoid that. So how do I make the PtLabel widget
“transparent for events” such that the mouse event actually goes through to
the PtGroup widget?