  is directly verifiable, as (1) in conjunction with O1 entails O3. S becomes indirectly verifiable, as O2 follows from S and (1), and (1) is directly verifiable. Should O2 follow from (1) alone, then O2 follows from O3&amp;¬S, which means that ¬S is directly verifiable (O2 and O3&amp;¬S being logically independent).