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.
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For example, the use of the 'medio-passive' in mental and targeting verbal clauses:
you were seen, you were accused
and in impersonal projection nexuses:
it was thought…, it was said… .