Halliday & Matthiessen (1999: 400-1):
A related textual system is that of THEME PREDICATION (the so-called "cleft construction" of formal grammar), which similarly employs the identifying type of clause to achieve a particular balance of meaning in the discourse. …A: It's the grammar where the fun is.B: Yes it's the grammar which is interesting.Here 'the thing which is interesting' is identified as grammar rather than lexis; the relationship between the grammar is interesting and it's the grammar which is interesting is shown in Figure 9-16.
