Halliday & Matthiessen (1999: 265):
Processes are realised by verbal groups, where, typically, the only lexical material is the verb itself, functioning as Event (material action or event, conscious or verbal process, or relation). Apart from the adverbial complement of a phrasal verb, which may serve to construct a distinct lexical item e.g. make out (I can’t make out the difference), come to (she’ll come to in a minute), let on (don’t let on about this), &c., all contrasts made by the verbal group are grammatical ones — tense and other quasi-temporal systems, and modality. There is no lexical expansion classifying processes into taxonomies or assigning them sets of contrasting qualities.