Halliday & Matthiessen (2014: 638-9):
Sometimes in a WH- clause, or its response, the Mood element is left in and only the Residue is ellipsed. For example, with WH- Subject:Has the time come for these local divinities [[to give way to perhaps a [ bigger concept of deity, a bigger concept of religion]]? – Who knows [∅: whether the time has come ...]?
And Hugo told you that, too. – Who did [∅: tell me that too]?
Similarly if the WH- element is part of the Residue:||| I think || that’s why my generation is so tediously over-serious. ||| How could we not be [∅: so tediously over-serious]? |||
Yes, I think you’d better look at it. – I don’t see any particular reason why I should [∅: look at it].
The Mood element may be represented by negative polarity alone:Yes, Dad, but we mustn’t even lean on this guitar today. – Why [∅: must we] not [∅: lean on this guitar today]?