So far we have addressed that when an event occurs, a fluent changes its truth value. What about the opposite? That is, when a fluent changes its truth value, an event occurs; more generally, when a particular condition becomes true, an event occurs. We call such an event a triggered event. An example of a triggered event is a ball bouncing off of a wall when it reaches the wall. This chapter addresses the triggering of events in response to conditions; we also discuss triggered fluents.
We specify when a triggered event occurs using a trigger axiom.
If γ is a condition, α is an event term, and τ is a timepoint term, then
is a trigger axiom.