Halliday & Matthiessen (1999: 227):
We have seen that sequences, figures, and elements are congruently realised in the grammar as follows:
clause complex clause group sequence ü figure ü element üBut these resources may be expanded by taking up further options in realisation; for example, sequences may alternatively be realised by clauses and even groups. This what we refer to as grammatical metaphor. Grammatical metaphor expands the semantic potential of the system.