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CHAPTER 4

# The Triggering of Events

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.

## 4.1 Trigger Axioms

We specify when a triggered event occurs using a trigger axiom.

##### DEFINITION 4.1

If γ is a condition, α is an event term, and τ is a timepoint term, then

is a trigger axiom.

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