Halliday & Matthiessen (1999: 125-6):
(ii) On the other hand, a clause complex may in principle correspond to a figure rather than a sequence. This happens through grammatical metaphor of an interpersonal kind (see Halliday, 1985: Section 10.4): an interpersonal modality that would be realised congruently as a modal auxiliary (can, may, will &c) or a modal adjunct (perhaps, probably &c) is 'upgraded' to the status of a projecting clause in a clause complex; for example:
I don't suppose || there's very much 'there is probably not'I think || I might have walked out too from all the accounts 'I probably might have'