 Thus, \(p\) is obligatory iff it holds in all the \(i\)-acceptable worlds, permissible iff it holds in some such world, impermissible iff it holds in no such world, omissible iff its negation holds in some such world, optional iff \(p\) holds in some such world, and so does \(\neg p\), and non-optional when \(p\) either holds at all such worlds or at none. If a formula is true at every world in any such model of serially-related worlds, then the formula is  valid.[32]