Halliday & Matthiessen (1999: 26):
… what makes such a theory (i.e. an ideation base as the construal of experience) possible is that it is a stratal construction that can be deconstructed, every such occasion being a gateway to the creation of further meanings which reconstrue in new and divergent ways. Thus a sequence is not ‘the same thing as’ a clause complex; if it was, language would not be a dynamic open system of the kind that it is.