The Trains project is an effort to build a conversationally proficient planning assistant. A key part of the project is the construction of the Trains system, which provides the research platform for a wide range of issues in natural language understanding, mixedinitiative planning systems, and representing and reasoning about time, actions and events. Four years have now passed since the beginning of the project. Each year we have produced a demonstration system that focused on a dialog that illustrates particular aspects of our research. The commitment to building complete integrated systems is a significant overhead on the research, but we feel it is essential to guarantee that the results constitute real progress in the field. This paper describes the goals of the project, and our experience with the effort so far. This paper is to appear in the Journal of Experimental and Theoretical AI, 1995. The TRAINS project has been funded in part by ONR/ARPA grant N00014-92-J-1512, U.S. Air ...

A class of interval-based temporal languages for uniformly representing and reasoning about actions and plans is presented. Actions are represented by describing what is true while the action itself is occurring, and plans are constructed by temporally relating actions and world states. The temporal languages are members of the family of Description Logics, which are characterized by high expressivity combined with good computational properties. The subsumption problem for a class of temporal Description Logics is investigated and sound and complete decision procedures are given. The basic language TL-F is considered #rst: it is the composition of a temporal logic TL # able to express interval temporal networks # together with the non-temporal logic F # a Feature Description Logic. It is proven that subsumption in this language is an NP-complete problem. Then it is shown how to reason with the more expressive languages TLU-FU and TL-ALCF . The former adds disjunction both at...

In connection with the DAML project for bringing about the Semantic Web, an ontology of time is being developed for describing the temporal content of Web pages and the temporal properties of Web services. The bulk of information on the Web is in natural language, and this information will be easier to encode for the Semantic Web insofar as community-wide annotation and automatic tagging schemes and the DAML time ontology are compatible with each other.

In this paper we provide a description of TimeML, a rich specification language for event and temporal expressions in natural language text, developed in the context of the AQUAINT program on Question Answering Systems. Unlike most previous work on event annotation, TimeML captures three distinct phenomena in temporal markup: (1) it systematically anchors event predicates to a broad range of temporally denotating expressions; (2) it orders event expressions in text relative to one another, both intrasententially and in discourse; and (3) it allows for a delayed (underspecified) interpretation of partially determined temporal expressions. We demonstrate the expressiveness of TimeML for a broad range of syntactic and semantic contexts, including aspectual predication, modal subordination, and an initial treatment of lexical and constructional causation in text. 1

We describe a procedure for arranging into a time-line the contents of news stories describing the development of some situation. We describe the parts of the system that deal with 1. breaking sentences into event-clauses and 2. resolving both explicit and implicit temporal references. Evaluations show a performance of 52%, compared to humans.

In this paper we develop a representation for the temporal structure inherent in human actions and demonstrate an effective method for using that representation to detect the occurrence of actions. The temporal structure of the action, sub-actions, events, and sensor information is described using a constraint network based on Allen's interval algebra. We map these networks onto a simpler, 3-valued domain (past,now,fut) network --- a PNF-network --- to allow fast detection of actions and sub-actions. The occurrence of an action is computed by considering the minimal domain of its PNF-network, under constraints imposed by the current state of the sensors and the previous states of the network. We illustrate the approach with examples, showing that a major advantage of PNF propagation is the detection and removal of situations inconsistent with the temporal structure of the action. We also examine a method to increase the robustness of PNF-propagation in the case of faulty sensors. 1 In...

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Contrary to cognitive approaches of agency where a lot of effort is devoted to the formalization of agent concepts, little work has been done on the formalization of situated multi-agent systems (situated MASs). In this paper we present a generic model for situated MASs. This model formally describes an abstract architecture for situated MASs. In this architecture each agent is situated in his local context that he is able to perceive and in which he can act. Since intelligence in situated MASs results from the interactions of agents with the environment rather than from their individual capabilities, the model takes an action-centric approach. The model deals with (1) the actions of agents in the environment, (2) ongoing activities in the environment, such as moving objects, and (3) the interactions between agents and ongoing activities through the environment. One model for

We compare two approaches to event definition, deriving from the Active Database and Knowledge Representation communities. We relate these approaches by taking a system of the former kind, displaying some of its shortcomings, and rectifying them by remodelling the system in the latter style. We further show the extent to which the original system can be recreated within the remodelled system. This bridge between the two approaches should provide a starting point for fruitful interaction between the two communities.

Natural language generation (NLG) is first and foremost a reasoning task. In this reasoning, a system plans a communicative act that will signal key facts about the domain to the hearer. In generating action descriptions, this reasoning draws on characterizations both of the causal properties of the domain and the states of knowledge of the participants in the conversation. This dissertation shows how such characterizations can be specified declaratively and accessed efficiently in NLG. The heart of this dissertation is a study of logical statements about knowledge and action in modal logic. By investigating the proof-theory of modal logic from a logic programming point of view, I show how many kinds of modal statements can be seen as straightforward instructions for computationally manageable search, just as Prolog clauses can. These modal statements provide sufficient expressive resources for an NLG system to represent the effects of actions in the world or to model an addressee whose knowledge in some respects exceeds and in other respects falls short of its own. To illustrate the use of such statements, I describe how the SPUD sentence planner exploits a modal knowledge base to