how to override the upper-right quitbutton?

[jca <jca mail phm vcu edu>]

  I have what must be a fairly common problem...

  When the user leaves my program, I must trap that and prompt
him/her/it to save their data before leaving.  That's easy to do from
the file->exit menu option, because I defined that myself...

  But when the user clicks on the X in the upper right corner of the
window, it seems all my GUI activity is shut down before I can do
anything.  And of course with no GUI, it can't prompt to save...
  (Right now I try to launch a save-dialogue from the main window's
"unrealize" event; too late.  The save-dialogue is never seen before
shutdown.)

  How do I catch an upper-right-button-initiated window shutdown
*before* my GUI is killed?  And then stall the window's death until it
can get a word in edgewise about saving data?

  Thanks,
  jca