in expressiveness. Current situation The latest version of the language is PDDL3.1. The BNF (Backus–Naur Form) syntax definition of PDDL3.1 can be found among the resources of the IPC-2011 homepage or the IPC-2014 homepage. Successors/variants/extensions of PDDL PDDL+ This extension of PDDL2.1 from around 2002–2006 provides a more flexible model of continuous change through the use of autonomous processes and events. The key this extension provides is the ability to model the interaction between the agent's behaviour and changes that are initiated by the agent's environment. Processes run over time and have a continuous effect on numeric values. They are initiated and terminated either by the direct action of the agent or by events triggered in the environment. This 3-part structure is referred to as the start-process-stop model. Distinctions are made between logical and numeric states: transitions between logical states are assumed to be instantaneous whilst occupation of a given logical state can endure over time. Thus in PDDL+ continuous update expressions are restricted to occur only in process effects. Actions and events, which are instantaneous, are restricted to the expression of discrete change. This introduces the before mentioned 3-part modelling of periods of continuous change: (1) an action or event starts a period of continuous change on a numeric variable expressed by means of a process; (2) the process realizes the continuous change of the numeric variable; (3) an action or event finally stops the execution of the process and terminates its effect on the numeric variable. Comment: the goals of the plan might be achieved before an active process is stopped. NDDL NDDL (New Domain Definition Language) is NASA's response to PDDL from around 2002. Its representation differs from PDDL in several respects: 1) it uses a variable/value representation (timelines/activities) rather than a propositional/first-order logic, and 2) there is no concept of states or actions, only of intervals (activities) and constraints between those activities. In this respect, models in NDDL look more like schemas for SAT encodings of planning problems rather than PDDL models. Because of the mentioned differences planning and execution of plans (e.g. during critical space missions) may be more robust when using NDDL, but the correspondence to standard planning-problem representations other than PDDL may be much less intuitive than in case of PDDL. MAPL MAPL (Multi-Agent Planning Language, pronounced "maple") is an extension of PDDL2.1 from around 2003. It is a quite serious modification of