Left the document, middle the units, right the associated text portions. A unit may be marked as included or ignored . This mark is called the unit status . When a unit is a descendant of any ignored unit, it is 'hidden'. In all other cases it is 'manifest'.
Units thus record the name of a status value, which itself is recorded by the document.
A unit that is ignored does not play a role in prediction. A unit may also be marked as temporary . In this case the unit maintains its status but may signal its temporary nature when required. Each unit is temporary when its parent is temporary, or when it is marked as temporary itself. A unit type may be breaking or continuous .