The paper discusses some properties of system descriptions in action language – a recent extension of action language by defined fluents. We give a sufficient condition guaranteeing that states of an system description are fully determined by statics and inertial fluents. In system descriptions satisfying this condition, defined fluents simply facilitate the description of dynamic domains; they are not essential and can be eliminated. We use our sufficient condition to identify a common core of action languages and. This is an (...) expansion of the work of, who identified a common core of languages and. The results presented in this paper are based on the close relationship between action languages and the theory of logic programs under the answer set semantics. (shrink)

“Toy worlds” involving actions, such as the blocks world and the Missionaries and Cannibals puzzle, are often used by researchers in the areas of commonsense reasoning and planning to illustrate and test their ideas. We would like to create a database of generalpurpose knowledge about actions that encodes common features of many action domains of this kind, in the same way as abstract algebra and topology represent common features of specific number systems. This paper is a report on the first (...) stage of this project—the design of an action description language in which this database will be written. The new language is an extension of the action language C+. Its main distinctive feature is the possibility of referring to other action descriptions in the definition of a new action domain. (shrink)