Halliday & Matthiessen (1999: 255):
Very often, however, in deriving any one element we should have to take two or more steps in the course of unpacking. For example, in the development of our understanding, we can unpack to our understanding develops. However, our understanding is itself a metaphorical entity consisting of a 'thing' derived from a process; and a 'possessor' derived from a participant; so it can be unpacked further to we understand, giving something like [the way] we understand develops. We then might want to consider develops as (be/ become) quality => process, so [the extent to which] we understand becomes greater, i.e. we understand more and more.