Tkinter - tracking toplevel windows

[Andrew Wheatley]

I have a Tkinter Gui with a button which generates other toplevel windows.
Whenever a toplevel is generated, I append its id to a global list.  What I'd like
to do with wm_protocol/WM_DELETE_WINDOW method is remove from the list
the window id currently being killed and then destroy the window.

My question is:

How can I tell which window is being destroy when I kill it with the X in the 
top right hand corner.