An event is either the expiration of a timer, or the matching
of some pattern in the input. Events that haven't happened yet are
pending events. Pending events are created with the
timer and match commands, respectively.
Each pending event has an associated label (called the target
label) of the event. When and if the event occurs, execution
resumes starting at the target label. Events only occur during
the execution of a wait command.
Events are grouped into named sets. Events in the same set are usually related, in the sense that they represent alternate outcomes to the same situation.
A set of pending events may be explicitly cancelled
(i.e., forgotten) before they occur with the cancel
command. All events in the set are cancelled.
The other way a set of pending events may be cancelled is implicitly, when any event in the set occurs. Execution resumes at the target label associated with the event that occurred, and all other events in that set are immediately cancelled.