Halliday & Matthiessen (1999: 156):
In the grammar therefore the Medium appears as an obligatory element — the only element that has this status in the clause. This does not mean that we will find a nominal reflex of the Medium made explicit in the syntagm of every clause; there are various ways in which the Medium may be present as a cryptotypic feature rather than as an overt form. Nevertheless its presence is required in some guise or other; and this distinguishes the Medium from all other participants in the figure.
Blogger Note:
it
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had been raining
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function structure
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Process/Medium
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syntagm
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nominal group
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verbal group
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But see Halliday & Matthiessen (2004: 289).