Martin (1992: 462):
The challenge here is to interpret the meaning of Subject from a discourse perspective, by looking at the way in which discourse structures interact with Subject selection.
 This confuses metafunctions. Subject is an interpersonal function whereas discourse semantics has been demonstrated here to be largely a misunderstanding of Halliday and Hasan's model of lexicogrammatical cohesion, a non-structural resource of the textual metafunction. The meaning of Subject is interpersonal, not textual. Halliday & Matthiessen (1999: 528):
The “textual” metafunction is the name we give to the systematic resources a language must have for creating discourse: for ensuring that each instance of text makes contact with its environment. The “environment” includes both the context of situation and other instances of text.
 This confuses axes, and therefore levels of symbolic abstraction, since syntagmatic structures realise paradigmatic selections.
 As has been previously demonstrated here, the discourse "structures" reworked from Halliday and Hasan's model of lexicogrammatical cohesion are not structures, but cohesive relations.