  The first three axioms state the conditions under which a fluent can hold at a time \(t\): either it holds right from the start or it is initiated at a time \(t_1\) before \(t\) without a terminating action occurring in between the two. The fourth axiom defines the predicate Clipped\((t_1, f, t_2)\) saying that a fluent \(f\) is clipped between times \(t_1\) and \(t_2\) if it is terminated by some action \(a\) happening between \(t_1\) and \(t_2\). Simplifying a whole lot, the axioms (30)–(33) formalize the notion of instantaneous change, such as two balls colliding or somebody dying. Moreover, they embody a notion of inertia: fluents continue to hold unless terminated.  