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Contributors: Adam Green,

PDDL+ (missing reference) introduced processes and events, to the domain of PDDL. Where previous updates to PDDL had introduced intractable rules such as Derived Predicates and Constraints, PDDL+ is the first to consider essentially actions which must be applied when their preconditions are met.

Processes directly correspond to a durative action and last for as long as their pre-condition is met. A process is something like gravity’s effect on a ball, increases the velocity of the ball until it either reaches terminal velocity or indeed, it hits the ground.

Events directly correspond to instantaneous actions, and happen the instant their preconditions are met, usually with the effect of transforming their state such that their precondition is no longer met. Events are uncontrollable, and in our ball example, we might consider an event to be the ball hitting the ground. In that instance, the velocity of the ball is negated, and multiplied by some bounce coefficient.

Processes and events are still very much a challenge for some planners, and support is somewhat patch, with certain planners losing support for earlier features in PDDL in order to support this new feature. Most notably, Processes can (and usually do) have a continuous effect in a state, and therefore cause problems if we define a preference such as preference (always (< (velocity ball) 10)).

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