Hierarchical Plans in a Description Logic of Time and Action

A formal language for representing and reasoning about time, actions and plans in a uniform way is presented. We employ an action representation in the style of Allen, where an action is represented by describing what is true while the action is occurring. In this sense, an action is defined by means of temporal constraints on the world states, which pertain to the action itself, and on other possible qualifications for the action occurring over time, while a plan is seen just as a complex action. Therefore, there is no difference between actions and plans in this framework. A distinction between action types and individual actions is supported by the formalism. In this paper, we show how to extend the language with a decomposition operator in a way of distinguishing the different actions composing a plan and not just the different temporal qualifications for the plan itself. Thus, a plan is a hierarchical structure made of its distinct component subparts. The formal representation language used in this paper is a member of the family of description logics, and it is provided with a well founded syntax, semantics and calculus. The classification and recognition tasks, together with the basic subsumption procedure, form the basis for action management. An action description can be automatically classified into a taxonomy; an action instance can be recognized to take place at a certain moment from the observation of what is happening in the world during a time interval