the original language. It introduces non-propositional state-variables (which may be n-ary: true, false, unknown, or anything else). It introduces a temporal model given with modal operators (before, after, etc.). Nonetheless, in PDDL3.0 a more thorough temporal model was given, which is also compatible with the original PDDL syntax (and it is just an optional addition). MAPL also introduces actions whose duration will be determined in runtime and explicit plan synchronization which is realized through speech act based communication among agents. This assumption may be artificial, since agents executing concurrent plans shouldn't necessarily communicate to be able to function in a multi-agent environment. Finally, MAPL introduces events (endogenous and exogenous) for the sake of handling concurrency of actions. Thus events become part of plans explicitly, and are assigned to agents by a control function, which is also part of the plan. OPT OPT (Ontology with Polymorphic Types) was a profound extension of PDDL2.1 by Drew McDermott from around 2003–2005 (with some similarities to PDDL+). It was an attempt to create a general-purpose notation for creating ontologies, defined as formalized conceptual frameworks for planning domains about which planning applications are to reason. Its syntax was based on PDDL, but it had a much more elaborate type system, which allowed users to make use of higher-order constructs such as explicit λ-expressions allowing for efficient type inference (i.e. not only domain objects had types (level 0 types), but also the functions/fluents defined above these objects had types in the form of arbitrary mappings (level 1 types), which could be generic, so their parameters (the domain and range of the generic mapping) could be defined with variables, which could have an even higher level type (level 2 type) not to speak of that the mappings could be arbitrary, i.e. the domain or range of a function (e.g. predicate, numeric fluent) could be any level 0/1/2 type. For example, functions could map from arbitrary functions to arbitrary functions...). OPT was basically intended to be (almost) upwardly compatible with PDDL2.1. The notation for processes and durative actions was borrowed mainly from PDDL+ and PDDL2.1, but beyond that OPT offered many other significant extensions (e.g. data-structures, non-Boolean fluents, return-values for actions, links between actions, hierarchical action expansion, hierarchy of domain definitions, the use of namespaces for compatibility with the semantic web). PPDDL PPDDL (Probabilistic PDDL) 1.0 was the official language of the probabilistic track of the 4th and