Halliday & Matthiessen (2014: 397):
The experiential structure of the finite verbal group is Finite (standing for ‘Finite operator’) plus Event, with optional elements Auxiliary (one or more) and Polarity. Finite verbal groups range from short, one-word items such as ate, where the Finite is fused with the Event and there is no Auxiliary, to long strings like couldn’t have been going to be being eaten.