Halliday & Matthiessen (2014: 639):
The elliptical or substitute clause requires the listener to ‘supply the missing words’; and since they are to be supplied from what has gone before, the effect is cohesive. It is always possible to ‘reconstitute’ the ellipsed item so that it becomes fully explicit. Since ellipsis is a lexicogrammatical resource, what is taken over is the exact wording, subject only to the reversal of speaker-listener deixis (I for you and so on), and change of mood where appropriate.