Halliday & Matthiessen (1999: 287-8):
The alternative is to try to conflate the two into a single representational phase. It is difficult to do this without making the result far too complicated; but an attempt is made in Figure 6-15. What this does is set up distinct junctional categories based on the types of grammatical metaphor … . This kind of approach is perhaps the one to be developed further, since it brings out more clearly the potential of grammatical metaphor as a semogenic resource. But we shall not attempt to pursue it further here.
