switch (line.vertical_position.reference.get_value_or(sub::BOTTOM_OF_SCREEN)) {
case sub::BOTTOM_OF_SCREEN:
case sub::TOP_OF_SUBTITLE:
switch (line.vertical_position.reference.get_value_or(sub::BOTTOM_OF_SCREEN)) {
case sub::BOTTOM_OF_SCREEN:
case sub::TOP_OF_SUBTITLE: