An hevent is referred to by Henzinger as an , but we have changed the name to avoid terminological confusion with events in PDDL+.
Figure 8 shows a very simple dynamic system expressed as a hybrid automaton.

The initial, jump and flow conditions are not necessarily satisfied by unique valuations. When multiple valuations satisfy these conditions it is possible for the behaviour of the automaton to be nondeterministic.
It should be observed that Henzinger's model needs to be extended, in a simple way, to include the undefined value, , for realvalued variables. This is because PDDL+ states can contain unassigned realvalued variables, as shown in Core Definition 2 which defines the metric valuation of a state. The role of this value is to allow for the situations in which a metric fluent is created in a domain, but is not given an initial value. Such a fluent will be given the undefined value and attempts to inspect it before it is assigned a value will be considered to yield an error. This introduces no semantic difficulties and we have left the details of this modification implicit.
Just as the input language of a finite automaton can be defined to be the set of all sequences of symbols from its alphabet, it is possible to define the input language of a Hybrid Automaton. In this case, the elements of the language are called traces:
Informally, a trace consists of a sequence of hevents interleaved with real values corresponding to time periods during which no hevents are applied. The time at which each hevent is applied is readily determined by summing the values of the time periods in the sequence up to the point at which the hevent appears: hevents take no time to execute. Note that this definition does not require that the trace is accepted by the Hybrid Automaton: this is a property of traces we consider in Section 6.4.
A further minor point to note is that the definition of a trace allows traces where the first transition occurs at time 0. Our convention in the semantics of PDDL2.1 is to forbid actions to occur at time 0. The reason for this is discussed in [Fox LongFox Long2003], but, briefly, in order to be consistent with the model in which states hold over an interval that is closed on the left and open on the right, the initial state (which holds at time 0) must persist for a nonzero interval. As a consequence, we will not be interested in traces that have action transitions at time 0.
Derek Long 20061009