Halliday & Matthiessen (2014: 176, 176n):
In non-finite clauses, … the not (or other negative modal Adjunct) may constitute a Mood element either on its own, or together with the Subject if there is one. … if the agnate finite clause is negative (as shown by the tag …) then the negative Adjunct functions as Mood element. If the agnate finite clause is positive … then the negative Adjunct forms part of the Residue.