the definition of all the possible objects (atoms in the logical universe), initial conditions (the initial state of the planning environment, a conjunction of true/false facts), and the definition of goal-states (a logical expression over facts that should be true/false in a goal-state of the planning environment). Thus eventually PDDL1.2 captured the "physics" of a deterministic single-agent discrete fully accessible planning environment. PDDL2.1 This was the official language of the 3rd IPC in 2002. It introduced numeric fluents (e.g. to model non-binary resources such as fuel-level, time, energy, distance, weight, ...), plan-metrics (to allow quantitative evaluation of plans, and not just goal-driven, but utility-driven planning, i.e. optimization, metric-minimization/maximization), and durative/continuous actions (which could have variable, non-discrete length, conditions and effects). Eventually PDDL2.1 allowed the representation and solution of many more real-world problems than the original version of the language. PDDL2.2 This was the official language of the deterministic track of the 4th IPC in 2004. It introduced derived predicates (to model the dependency of given facts from other facts, e.g. if A is reachable from B, and B is reachable from C, then A is reachable from C (transitivity)), and timed initial literals (to model exogenous events occurring at given time independently from plan-execution). Eventually PDDL2.2 extended the language with a few important elements, but wasn't a radical evolution compared to PDDL2.1 after PDDL1.2. PDDL3.0 This was the official language of the deterministic track of the 5th IPC in 2006. It introduced state-trajectory constraints (hard-constraints in form of modal-logic expressions, which should be true for the state-trajectory produced during the execution of a plan, which is a solution of the given planning problem) and preferences (soft-constraints in form of logical expressions, similar to hard-constraints, but their satisfaction wasn't necessary, although it could be incorporated into the plan-metric e.g. to maximize the number of satisfied preferences, or to just measure the quality of a plan) to enable preference-based planning. Eventually PDDL3.0 updated the expressiveness of the language to be able to cope with recent, important developments in planning. PDDL3.1 This was the official language of the deterministic track of the 6th and 7th IPC in 2008 and 2011 respectively. It introduced object-fluents (i.e. functions' range now could be not only numerical (integer or real), but it could be any object-type also). Thus PDDL3.1 adapted the language even more to modern expectations with a syntactically seemingly small, but semantically quite significant change