Martin (1992: 388):
As Halliday and Hasan point out, this redundancy is typically exploited in the context of repudiation. The second response in [6:3] illustrates this function. The Thing drink is presumed from the offer by the substitute one; this allows the unmarked tonic to fall on small, which is contrastive in this environment.
[6:3] Would you like a drink? a drink — //13 A small gin and tonic thanks // # — //13 Just a small one thanks // one — //13 Just the smaller one thanks //
Note the absence of repudiation in the first response, where gin and tonic is realised explicitly, proposed as a hyponym of drink. The third response illustrates the difference between repudiation and comparison; one presumes the Thing drink as in the second response, but in addition smaller presumes a set of bigger drinks which are not identified in the second.
 Contrary to what is implied here, the discussion in Halliday & Hasan (1976: 93-5) of repudiation in substitution–&–ellipsis does not conceive it in terms of redundancy, and so does not support Martin's interpretation. [Note the omission of page references.]
 This discussion completely misunderstands the notion of repudiation, and the text does not illustrate it. Cf. Halliday & Hasan (1976: 93):
The notion of repudiation is explained as follows. In any anaphoric context, something is carried over from a previous instance. What is carried over may be the whole of what there was, or it may be only a part of it; and if it is only a part of it, then the remainder, that which is not carried over, has to be REPUDIATED. For example, in
[3:10] We have no coal fires; only wood ones.
fires is carried over anaphorically, but coal is repudiated.
 It is not true that using the substitute one 'allows the unmarked tonic to fall on small. The tonic falls on small to mark contrastive prominence. If the substitute had not been used, the tonic would still fall on small, in the unmarked case.
 There is no repudiation in any of the responses, because there is nothing to repudiate in the offer. See  above.
 Since there is no repudiation in the third response, it does not illustrate the difference between repudiation (substitution–&–ellipsis) and comparison (reference).