Halliday &
Matthiessen (2004: 419):
Adverbial conjunctions are as/so long as, as/so far as, (as) much as, for example as long as you’re here …, as far as I know …, much as I’d like to … (compare non-finite as well as, which is extending not enhancing. In origin these express limitation, a particular point up to which a certain circumstance is valid.