Martin (1992: 445):
Theme has been analysed in dependent clauses with Subjects and β clauses have been interpreted as marked Themes where they are realised before their α, ignoring Themes in this α as above; Predicators in imperative and non-finite clauses are not treated as Themes, following Halliday (1985).
 A dependent clause in a 'regressive sequence' (β^α) clause nexus is not a marked topical Theme of a clause. It has thematic status within the clause nexus — not within a clause. See Halliday & Matthiessen (2004: 392-3).
 In such cases, to ignore the Theme of either clause in the nexus, α or β, is to misrepresent the data. Halliday & Matthiessen (2004: 394):
… the point to bear in mind is that there will be two thematic domains — that of the clause nexus, and that of the clause.
 In imperative clauses, 'it is the Predicator that is the unmarked Theme' (Halliday 1985: 49; Halliday 1994: 47; Halliday & Matthiessen 2004: 76; Halliday & Matthiessen 2014: 103).