code: def on_quit(self, event, wind=None):
          
          if wind:
              wind.Destroy()
          if not self.parent.IsShown():
              self.on_show_mainframe(None)
              # re-do the quit binding
              self.parent.Bind(wx.EVT_MENU, self.on_quit, self.file_quit)
          else:
              self.parent.Close()
docs: shut down application if in the main frame.
          otherwise, destroy the top window (wind) and restore
          the main frame.