Halliday & Matthiessen (1999: 280):
The following are some of the factors to be taken into account when we try to represent examples of semantic structural configurations:(i) Do we represent the text as it is (metaphorically)?(ii) Do we represent the text in "unpacked" form (congruently)?(iii) How far do we unpack it (move towards congruence)?(iv) If we give two (or more) representations, what relationship do we show between them (how do we show the agnation between congruent and metaphorical)?