Slave.set_failure_hook (Window.failure ws);
Slave.set_busy_hook (Window.throbber_busy ws);
Slave.set_idle_hook (Window.throbber_idle ws);
Slave.set_failure_hook (Window.failure ws);
Slave.set_busy_hook (Window.throbber_busy ws);
Slave.set_idle_hook (Window.throbber_idle ws);