Assert the radio button with the given label (recommended), name or id is
    chosen.

def assert_radio_selected(self, value):
    """
    Assert the radio button with the given label (recommended), name or id is
    chosen.
    """
    radio = find_field(world.browser, 'radio', value)
    assert radio, "Cannot find a '{}' radio button.".format(value)
    assert radio.is_selected(), "Radio button should be selected."