Halliday & Matthiessen (1999: 24):
The production of discourse by an individual speaker or writer can be seen as a dialectic between these two semiotic activities: between (i) recycling elements, figures and sequences that that individual has used many times before, and so for him or her are already fully codified, and (ii) constructing new ones that are being codified for the first time (and some of which may remain codified for future use — especially with a child who is learning the system).