Halliday & Matthiessen (1999: 22-3):
Semogenic processes of the kinds just described take place in all three dimensions of semohistory: as the system of language evolves, as children develop their language, and as the language of a text unfolds. Hence language embodies the potential for its own ongoing expansion; and since the system at any moment is the repository of its own history, we can sometimes recognise disjunctions or interstices that offer a likely context for new meanings to appear. For example, the 'double -ing ' form of the English verb, which has recently been establishing itself (e.g. being raining, as in it seemed better to stay at home with it being raining), could have been predicted from a knowledge of the present state and recent history of the tense system. A change of this kind will propagate steadily throughout the system: sometimes very rapidly, but more often in an irregular and rather uneven flow.
Let us refer to this process as that of codifying, noting that as always it is at once both semantic and lexicogrammatical: there is no implication that meanings are already there and waiting to be codified.