Halliday & Matthiessen (1999: 215):
… in any given language, (i) one or other parameter may be given prominence, (ii) two or more parameters may be combined into a single semantic system, (iii) any parameter may be construed either more grammatically or more lexically, and (iv) a number of features that are not strictly temporal may be incorporated into the picture, both ideational ones like attempting/succeeding and interpersonal ones like the speaker’s angle on the process — judgement of its likelihood, desirability, and so on.