Halliday & Matthiessen (1999: 117):
Expansion can be thought of as construing another dimension of experience, so that superimposed, as it were, on the construal of a figure — a basic fragment of experience in the form of a quantum of change (event, action, behaviour &c.) — is the construal of a nexus between two figures, such that one such fragment is non-randomly (i.e. meaningfully) cumulated with another.