Halliday & Matthiessen (2014: 475):
The hypotactic form of the alternative relation is if ... not (i.e. ‘if not a, then b’, with the dependent clause typically coming first). For example,||| If they’re not in their usual place || they could have fallen through onto the – |||
||| If it doesn’t come from [[ what’s outside us]] , from our experience, || it’s got to come from our inner nature. |||
||| If you haven’t lost it, || then it’s in that cupboard. |||
‘either you’ve lost it, or else it’s in that cupboard’. Either clause can be construed as the negative condition; we could just as well say if it’s not in that cupboard then you’ve lost it, the only difference being which one is chosen as Theme.