Halliday & Matthiessen (1999: 21-2):
A common variant of this process is that of the dissociation of associated features in the wording. We could represent this as:
Here again one meaning has been replaced by three: we now have (say) question₁ ↘ interrogative x rising tone, question₂ ↘ interrogative x falling tone, and question₃ ↘ declarative x rising tone, e.g. is she cóming? is she còming? she's cóming?