Halliday & Matthiessen (1999: 117):
From one standpoint the limiting case of expansion would be accumulation in a temporal sequence (hence our general term “sequence” for the product of this construal): ‘a happens, then x happens’. This gives value to a as the temporal circumstance of x. From this we could derive a wide range of more complex enhancing relations: variations on the simple temporal sequence (‘after’, ‘before’, ‘at the same time’, ‘immediately after’, &c.) and further circumstances such as cause, condition, concession and their subcategories. […] These are ‘enhancements’, multiplying one figure by another, as it were.