 These clauses are analogous to the clauses which define the modal operators \(\Box\) and \(\Diamond\)—with \(\bG\) and \(\bH\) corresponding to \(\Box\), and \(\bF\) and \(\bP\) to \(\Diamond\)—but instead of possible worlds the valuation function takes moments of time as arguments. Therefore principle (5) is valid in any model, since the modal analogue is valid for \(\Box\) independently of the accessibility relation. 