Halliday & Matthiessen (1999: 117):
extension is an additive relation between figures: a sequence is made bigger by the addition of another figure. This may involve pure addition (‘and’: he is too young and he doesn’t speak the language) or addition with an adversative feature (‘and yet’: he speaks the language but he is too young). As a variant of addition, we also have alternation (he is too young or else he is just immature).