Halliday & Matthiessen (2014: 637):
In addition, the substitute not is used when the answer is qualified by a negative in some way:
||| Is that [[ what it really is about]] , a cock and a fox? ||| – ||| No, not really. |||
where a positive clause is simply presupposed by ellipsis:
||| Did you feel || that you were taking a risk || in being so open about [[ what you were doing]] ? ||| – ||| Oh, sure, in some ways. |||