In Chapter 4 we formulated several knowledge representation and problem solving domains using AnsProlog* and focused on the program development aspect. In this chapter we consider reasoning about actions in a dynamic world and its application to plan verification, simple planning, planning with various kinds of domain constraints, observation assimilation and explanation, and diagnosis. We do a detailed and systematic formulation – in AnsProlog* – of the above issues starting from the simplest reasoning about action scenarios and gradually increasing its expressiveness by adding features such as causal constraints, and parallel execution of actions. We also prove properties of our AnsProlog* formulations using the results in Chapter 3.

Our motivation behind the choice of a detailed formulation of this domain is twofold. (i) Reasoning about actions captures both major issues of this book: knowledge representation and declarative problem solving. To reason about actions we need to formulate the frame problem whose intuitive meaning is that objects in the worlds do not normally change their properties. Formalizing this has been one of the benchmark problems of knowledge representation and reasoning formalisms. We show how AnsProlog* is up to this task. Reasoning about actions also forms the ground work for planning with actions, an important problem solving task. We present AnsProlog encodings of planning such that the answer sets each encode a plan. (ii) Our second motivation is in regard to the demonstration of the usefulness of the results in Chapter 3.