Proceedings of the AAAI Conference on Artificial Intelligence, 5
We consider structures of actions which are partially ordered for time, which may occur in parallel, and which have lasting effects on the state of the world. Such action structures are of interest for problem-solving with multiple actors, and for understanding narrative texts where several things are going on at the same time. They are also of interest for other branches of computer science besides AI. Actions in the action structure are characterized in terms of preconditions, postconditions, and prevail conditions, where the prevail condition is a requirement on what must hold for the duration of the action. All three conditions are partial states of the world, and therefore elements of a lattice. We develop the formalism, give an example, and specify formally the criterion for admissible action' structures, where postconditions of earlier actions serve as prevail- or preconditions of later actions in a coherent way, and there are no conflicting attempts to change ("update" a feature in the world.