Event operations evaluate, compare, and count events. They include the following operations:
Filter evaluates the current to determine whether they could potentially trigger a rule to fire
Window compares the current event to past events that have been stored in memory
Trigger counts events to determine whether enough events have occurred to trigger a rule
Each operation works on a set of events, receiving a set of events as input and returning a set of events as output. The current event processed by a rule often has a special meaning for the semantic of the language. The current event is always part of the set of events in and out of an operation unless the set is empty. If an input set of an operation is empty, then the operation is not evaluated.