Toggle all the output devices. For each device, if it's on, turn it
        off; if it's off, turn it on.

def toggle(self):
        """
        Toggle all the output devices. For each device, if it's on, turn it
        off; if it's off, turn it on.
        """
        for device in self:
            if isinstance(device, (OutputDevice, CompositeOutputDevice)):
                device.toggle()