Halliday & Matthiessen (1999: 126):
(in) In addition, one or more of the figures in the clause may be realised by something 'less than' a clause. One major source of this is ideational grammatical metaphor. But there are also other cases such as circumstances of Rôle with temporal implications — for example:
as a child, he was very shy 'when he was a child, he was very shy'.
The grammar forms complexes at ranks below the clause, of course: melted butter, a few shreds of lemon rind, a squeeze of lemon juice and chopped parsley. It is always possible to interpret certain instances of these as sequences that have been 'shrunk' by the grammar because they share one or more elements. Thus Henry and Anne went to the store might be interpreted as a simple figure if they went to the store together but as a sequence if they went there separately.