Gtk::Notebook: Signal to capture/veto page change event?

[Paul Goins <general vultaire net>]

I'm trying to write some code which will veto a Gtk::Notebook page
change under certain conditions (e.g. the user didn't press an "Apply"
button to confirm changes they made).  However, I'm not seeing any
signals for handling this in the online docs.  The closest thing I see
is a signal_switch_page, but that's not what I'm looking for since it
doesn't let me veto the change.  Any ideas?