Halliday & Matthiessen (1999: 105-6):
In a restricted register, such as those of weather forecasts and recipes, it may be possible to define sequential relations more precisely; but that is only because these registers are special cases, rather in the same way that the applications of propositional logic are special cases.
One might try to characterise sequences by reference to propositional logic; this would give some indication of what type of knowledge representation system is embodied in this domain of ideational semantics. However, such a characterisation would be likely to distort our understanding of sequences, because sequences cover and organise a considerably larger domain of the ideational semantic space than propositional logic attempts to do.