[…] the Theme of a clause consists of just one element, and that element is represented by just one constituent [...] A common variant of this elementary pattern is that in which the Theme consists of two or more elements forming a single complex element. Any element of clause structure may be represented by two or more groups or phrases forming a ʻcomplexʼ (group complex or phrase complex). Such a ʻcomplexʼ functions as a Theme in the normal way [...] Such Themes are still within the category of ʻsimpleʼ Themes. Any group complex or phrase complex constitutes a single element within the clause [...].
- Halliday (1985), a pag.40-41