Halliday & Matthiessen (1999: 572):
There will always be some complementarity of function between the more designed varieties and those that are naturally evolving. They may be allocated to different spheres of activity: for example, the language of bird–watchers vs the language of ornithologists. But in other cases the two are closely integrated as submotifs within a single sphere: for example, the use of both natural language and mathematical expressions side by side in the learning and practice of mathematics. This kind of interpenetration still entails a semiotic complementarity, but of a very sensitive kind, requiring a delicate interpretation of the context in order to bring it out.