 Note that this definition fails on reflexive temporal orders, which shows that the irreflexive versions of \(S\) and \(U\) are more expressive than their reflexive  counterparts.[4]  Other natural operators can be defined in terms of \(S\) and \(U\) as well, such as for instance Before, given by \(\varphi B\psi :=\lnot ((\lnot \varphi)U\psi)\) with intuitive reading “If \(\psi\) will be the case in the future, \(\varphi\) will occur before \(\psi\)”.