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...f the process will be known with certainty during plan execution. In the mdp framework, the agent is informed of the current state each time it takes an action. In many classical planners (e.g., snlp =-=[32]-=-, ucpop [38]), the current state can be calculated trivially from the known initial state and knowledge of the deterministic operators. The assumption of perfect knowledge is not valid in many domains...

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...nated planning research in the late 1980s and early 1990s, when classical planning was mainly characterised by the exploration of partial plan spaces, in planners such as tweak (Chapman, 1987), snlp (=-=McAllester & Rosenblitt, 1991-=-) and ucpop (Penberthy & Weld, 1992). Partial plans include a collection of actions representing the activity thus far determined to be part of a possible plan and a set of temporal constraints on tho...

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...e on the basis of certain assumptions. Virtually no plan is foolproof, and it is important that a formalism makes the necessary assumptions explicit so that they can be considered in evaluating plans. Our aim is to develop a general representation of actions and events that supports a wide range of reasoning tasks, including planning, explanation, and prediction, but also natural language understanding, and commonsense reasoning in general. Most of the previous work on these problems tends to address only a subset of these problems. The STRIPS-based planners (e.g., TWEAK [11], SIPE [69], SNLP [41]), for instance, only address the planning problem, while work in the situation calculus (e.g., [43, 9]) has primarily focussed on the prediction problem or on using it as an abstract theory for planning (e.g., [25]). Natural language work, on the other hand, typically only deals with commonsense entailments from statements about actions and events, sometimes with a focus on plan recognition and explanation (e.g., [4, 30, 57]). Our representation is intended to serve as a uniform representation to support all these tasks. As a result, we try to avoid introducing any specific syntactic construc...

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...e presented as a totally ordered sequence of operators (as in Strips (Fikes and Nilsson, 1971)) or as a partially ordered set of operators (as in Tweak (Chapman, 1987), NONLIN (Tate, 1977), and SNLP (=-=McAllester and Rosenblitt, 1991-=-)); the operators of the plan may be instantiated (e.g. in NOLIMIT (Veloso, 1989)) or contain variables with codesignations (e.g. in Tweak); the relations between operators and the goals they establis...