 In contrast, \(\OB\)-U, the axiom \(\OB (\OB p \rightarrow p)\) that we added to SDL to get SDL+, is not valid in the class of serial models. In order to validate \(\OB\)-U, we need the further requirement of “secondary seriality”: that any \(i\)-acceptable world, \(j\), must be in turn acceptable to itself. We can illustrate such an \(i\) and \(j\) as follows: