Derek Long

Bio: Derek Long is an academic researcher from King's College London. The author has contributed to research in topics: Domain (software engineering) & Automated planning and scheduling. The author has an hindex of 39, co-authored 196 publications receiving 7333 citations. Previous affiliations of Derek Long include Durham University & University College London.

PDDL2.1: an extension to PDDL for expressing temporal planning domains

Abstract: In recent years research in the planning community has moved increasingly towards application of planners to realistic problems involving both time and many types of resources. For example, interest in planning demonstrated by the space research community has inspired work in observation scheduling, planetary rover exploration and spacecraft control domains. Other temporal and resource-intensive domains including logistics planning, plant control and manufacturing have also helped to focus the community on the modelling and reasoning issues that must be confronted to make planning technology meet the challenges of application. The International Planning Competitions have acted as an important motivating force behind the progress that has been made in planning since 1998. The third competition (held in 2002) set the planning community the challenge of handling time and numeric resources. This necessitated the development of a modelling language capable of expressing temporal and numeric properties of planning domains. In this paper we describe the language, PDDL2.1, that was used in the competition. We describe the syntax of the language, its formal semantics and the validation of concurrent plans. We observe that PDDL2.1 has considerable modelling power -- exceeding the capabilities of current planning technology -- and presents a number of important challenges to the research community.

The 3rd international planning competition: results and analysis

Abstract: This paper reports the outcome of the third in the series of biennial international planning competitions, held in association with the International Conference on AI Planning and Scheduling (AIPS) in 2002. In addition to describing the domains, the planners and the objectives of the competition, the paper includes analysis of the results. The results are analysed from several perspectives, in order to address the questions of comparative performance between planners, comparative difficulty of domains, the degree of agreement between planners about the relative difficulty of individual problem instances and the question of how well planners scale relative to one another over increasingly difficult problems. The paper addresses these questions through statistical analysis of the raw results of the competition, in order to determine which results can be considered to be adequately supported by the data. The paper concludes with a discussion of some challenges for the future of the competition series.

Forward-chaining partial-order planning

Abstract: Over the last few years there has been a revival of interest in the idea of least-commitment planning with a number of researchers returning to the partial-order planning approaches of UCPOP and VHPOP. In this paper we explore the potential of a forward-chaining state-based search strategy to support partial-order planning in the solution of temporal-numeric problems. Our planner, POPF, is built on the foundations of grounded forward search, in combination with linear programming to handle continuous linear numeric change. To achieve a partial ordering we delay commitment to ordering decisions, timestamps and the values of numeric parameters, managing sets of constraints as actions are started and ended. In the context of a partially ordered collection of actions, constructing the linear program is complicated and we propose an efficient method for achieving this. Our late-commitment approach achieves flexibility, while benefiting from the informative search control of forward planning, and allows temporal and metric decisions to be made — as is most efficient — by the LP solver rather than by the discrete reasoning of the planner. We compare POPF with the approach of constructing a sequenced plan and then lifting a partial order from it, showing that our approach can offer improvements in terms of makespan, and time to find a solution, in several benchmark domains.

Plan Constraints and Preferences in PDDL3

Abstract: We propose an extension to the PDDL language, called PDDL3.0, that aims at a better characterization of plan quality by allowing the user to express strong and soft constraints about the structure of the desired plans, as well as strong and soft problem goals. PDDL3.0 was the reference language of the 5th International Planning competition (IPC-5). This paper contains most of the document about PDDL3.0 that was discussed by the Consulting Committee of IPC-5, and then distributed to the IPC-5 competitors. Introduction The notion of plan quality in automated planning is a practically very important issue. In many real-world planning domains, we have to address problems with a large set of solutions, or with a set of goals that cannot all be achieved. In these problems, it is important to generate plans of good or optimal quality achieving all problem goals (if possible) or some subset of them. In the previous International planning competitions, the plan generation CPU-time played a central role in the evaluation of the competing planners. In the fifth International planning competition (IPC-5), while considering the CPUtime, we would like to give greater emphasis to the importance of plan quality. The versions of PDDL used in the previous two competitions (PDDL2.1 and PDDL2.2) allow us to express some criteria for plan quality, such as the number of plan actions or parallel steps, and relatively complex plan metrics involving plan makespan and numerical quantities. These are powerful and expressive in domains that include metric fluents, but plan quality can still only be measured by plan size in the case of propositional planning. We believe that these criteria are insufficient, and we propose to extend PDDL with new constructs increasing its expressive power about the plan quality specification. The proposed extended language allows us to express strong and soft constraints on plan trajectories (i.e. constraints over possible actions in the plan and intermediate states reached by the plan), as well as strong and soft problem goals (i.e. goals that must be achieved in any valid plan, and goals that we desire to achieve, but that do not have to be necessarily achieved). Strong constraints and goals must be satisfied by any valid plan, while soft constraints and goals express desired constraints and goals, some of which may be more preferred than others. Informally, in planning with soft constraints and goals, the best quality plan should satisfy “as much as possible” the soft constraints and goals according to the specified preference relation distinguishing alternative feasible plans (satisfying all strong constraints and goals). While soft constraints have been extensively studied in the CSP literature, only very recently has the planning community started to investigate them (Brafman & Chernyavsky 2005; Briel et al. 2004; Delgrande, Schaub, & Tompits 2005; Miguel, Jarvis, & Shen 2001; Smith 2004; Son & Pontelli 2004), and we believe that they deserve more research efforts. The following are some informal examples of plan trajectory constraints and soft goals. Additional formal examples will be given in the next section. Examples in a blocksworld domain: a fragile block can never have something above it, or it can have at most one block on it; we would like that the blocks forming the same tower always have the same colour; in some state of the plan, all blocks should be on the table. Examples in a transportation domain: we would like that every airplane is used (instead of using only a few airplanes, because it is better to distribute the workload among the available resources and limit heavy usage); whenever a ship is ready at a port to load the containers it has to transport, all such containers should be ready at that port; we would like that at the end of the plan all trucks are clean and at their source location; we would like no truck to visit any destination more than once. When we have soft constraints and goals, it can be useful to give different priorities to them, and this should be taken into account in the plan quality evaluation. While there is more than one way to specify the importance of a soft constraint or goal, as a first attempt to tackle this issue, for IPC5 we have chosen a simple quantitative approach: each soft constraint and goal is associated with a numerical weight representing the cost of its violation in a plan (and hence also its relative importance with respect the other specified soft constraints and goals). Weighted soft constraints and goals are part of the plan metric expression, and the best quality plans are those optimising such an expression (more details are given in the next sections). Using this approach we can express that certain plans are more preferred than others. Some examples are (other formalised examples are given in the next sections):1 I prefer a plan where every airplane is used, rather than a plan using 100 units of fuel less, which could be expressed by weighting a failure to use all the planes by a number 100 times bigger than the weight associated with the fuel use in the plan metric; I prefer a plan where each city is visited at most once, rather than a plan with a shorter makespan, which could be expressed by using constraint violation costs penalising a failure to visit each city at most once very heavily; I prefer a plan where at the end each truck is at its start location, rather than a plan where every city is visited by at most one truck, which could be expressed by using goal costs penalising a goal failure of having every truck at its start location more heavily than a failure of having in the plan every city visited by at most one truck. We also observe that the rich additional expressive power we propose to add for goal specifications allows the expression of constraints that are actually derivable necessary properties of optimal plans. By adding them as goal conditions, we have a way to express constraints that we know will lead to the planner finding optimal plans. Similarly, one can express constraints that prevent a planner from exploring parts of the plan space that are known to lead to inefficient performance. In the next sections, we outline some extensions to PDDL2.2 that we propose for IPC-5. We call the extended language PDDL3.0. It should be noted that this is a preliminary version of the extended language, and that a more detailed description will be prepared in the future. Moreover, given that the proposed extensions are relatively new in the planning community, and that the teams participating in IPC-5 will have limited time to develop their systems, we impose some simplifying restrictions to make the language more accessible. State Trajectory Constraints Syntax and Intended Meaning State trajectory constraints assert conditions that must be met by the entire sequence of states visited during the execution of a plan. They are expressed through temporal modal operators over first order formulae involving state predicates. We recognise that there would be value in also allowing propositions asserting the occurrence of action instances in a plan, rather than simply describing properties of the states visited during execution of the plan, but we choose to restrict ourselves to state predicates in this extension of the language. The use of the extensions described here imply a new requirements flag, :constraints. The basic modal operators we propose to use in IPC-5 are: always, sometime, at-most-once, and atend (for goal state conditions). We use a special default assumption that unadorned conditions in the goal specification are automatically taken to be “at end” conditions. This assumption The benchmark domains and problems of IPC-5 contain many additional examples; some samples of them are described in (Gerevini & Long 2006). is made in order to preserve the standard meaning for existing goal specifications, despite the fact that in a standard semantics for an LTL formula an unadorned proposition would be interpreted according to the current state. We add within which can be used to express deadlines. In addition, rather than allowing arbitrary nesting of modal operators, we introduce some specific operators that offer some limited nesting. We have sometime-before, sometime-after, always-within. Other modalities could be added, but we believe that these are sufficiently powerful for an initial level of the sublanguage modelling constraints. It should be noted that, by combining these modalities with timed initial literals (defined in PDDL2.2), we can express further goal constraints. In particular, one can specify the interval of time when a goal should hold, or the lower bound on the time when it should hold. Since these are interesting and useful constraints, we introduce two modal operators as “syntactic sugar” of the basic language: hold-during and hold-after. Trajectory constraints are specified in the planning problem file in a new field, called :constraints that will usually appear after the goal. In addition, we allow constraints to be specified in the action domain file on the grounds that some constraints might be seen as safety conditions, or operating conditions, that are not physical limitations, but are nevertheless constraints that must always be respected in any valid plan for the domain (say legal constraints or operating procedures that must be respected). This also uses a section labelled (:constraints ...). The interpretation of (:constraints ...) in the conjunction of a domain and a problem file is that it is equivalent to having all the constraints added to the goals. The use of trajectory constraints (in the domain file or in the goal specification) implies the need for the :constraints flag in the :requirements list. Note that no temporal modal operator is allowed in preconditions of actions. That is, all action preconditions are with respect to a state (or time interval, in the case of overall action c

Intelligent Agents: Theory and Practice

Abstract: The concept of an agent has become important in both Artificial Intelligence (AI) and mainstream computer science. Our aim in this paper is to point the reader at what we perceive to be the most important theoretical and practical issues associated with the design and construction of intelligent agents. For convenience, we divide these issues into three areas (though as the reader will see, the divisions are at times somewhat arbitrary). Agent theory is concerned with the question of what an agent is, and the use of mathematical formalisms for representing and reasoning about the properties of agents. Agent architectures can be thought of as software engineering models of agents;researchers in this area are primarily concerned with the problem of designing software or hardware systems that will satisfy the properties specified by agent theorists. Finally, agent languages are software systems for programming and experimenting with agents; these languages may embody principles proposed by theorists. The paper is not intended to serve as a tutorial introduction to all the issues mentioned; we hope instead simply to identify the most important issues, and point to work that elaborates on them. The article includes a short review of current and potential applications of agent technology.

Introduction to Multiagent Systems

Abstract: From the Publisher: An agent is an entity with domain knowledge, goals and actions. Multi-agent systems are a set of agents which interact in a common environment. Multi-agent systems deal with the construction of complex systems involving multiple agents and their coordination. A multi-agent system (MAS) is a distributed computing system with autonomous interacting intelligent agents that coordinate their actions so as to achieve its goal(s) jointly or competitively.

World energy outlook

Abstract: This chapter discusses leading problems linked to energy that the world is now confronting and to propose some ideas concerning possible solutions. Oil deserves special attention among all energy sources. Since the beginning of 1981, it has merely been continuing and enhancing the downward movement in consumption and prices caused by excessive rises, especially for light crudes such as those from Africa, and the slowing down of worldwide economic growth. Densely-populated oil-producing countries need to produce to live, to pay for their food and their equipment. If the economic growth of the industrialized countries were to be 4%, even if investment in the rational use of energy were pushed to the limit and the development of nonpetroleum energy sources were also pursued actively, it would be extremely difficult to prevent a sharp rise in prices. It is evident that it is absolutely necessary to pursue actively the development of coal, natural gas, and nuclear power if a physical shortage of energy is not to block economic growth.

Peeking Inside the Black-Box: A Survey on Explainable Artificial Intelligence (XAI)

Abstract: At the dawn of the fourth industrial revolution, we are witnessing a fast and widespread adoption of artificial intelligence (AI) in our daily life, which contributes to accelerating the shift towards a more algorithmic society. However, even with such unprecedented advancements, a key impediment to the use of AI-based systems is that they often lack transparency. Indeed, the black-box nature of these systems allows powerful predictions, but it cannot be directly explained. This issue has triggered a new debate on explainable AI (XAI). A research field holds substantial promise for improving trust and transparency of AI-based systems. It is recognized as the sine qua non for AI to continue making steady progress without disruption. This survey provides an entry point for interested researchers and practitioners to learn key aspects of the young and rapidly growing body of research related to XAI. Through the lens of the literature, we review the existing approaches regarding the topic, discuss trends surrounding its sphere, and present major research trajectories.