Halliday & Matthiessen (2004: 358-9):
Within the 'primary' word class of adverbials, there is another class besides adverbs, namely conjunctions. […] they form three subclasses, namely linker, binder and continuative. Conjunctions also form word groups by modification, for example even if, just as, not until, if only. These can be represented in the same way, as b^a structures (or a^b in the case of if only). Note, however, that many conjunctive expressions have evolved from more complex structures, eg as soon as, in case, by the time, nevertheless, in so far as. These can be treated as single elements without further analysis. They are themselves, of course, subject to modification, eg just in case, almost as soon as.