Polarity In Mood vs Residue

Halliday & Matthiessen (2014: 175):

Finally, we should note that the negative word not occurs in two functions: either it is simply a formal or written variant of the Finite negative element n’t, in which case it is part of the Finite; or it is a distinct modal Adjunct in Mood or Residue. In the latter case it is phonologically salient and may also be tonic…

Tagged Imperative Or Metaphorical Interrogative?

The first analysis treats this instance as a tagged imperative clause, whereas the second treats it as an interpersonal metaphor of mood: a command realised as polar interrogative clause. Cf

This raises the question as to whether the tagging of the imperative facilitated the genesis of the metaphorical variant.