In artificial intelligence, a method is provided for representing a directed acyclic graph of worlds using an assumption-based truth maintenance system (ATMS) as a tool. The invention introduces the concepts of a nondeletion assumption and a deletion nogood, in order to allow deletion of an assertion upon transition between worlds. The traditional (de Kleer) ATMS tool is augmented to allow distinction between two kinds of assumptions, namely the nondeletion assumption and the world assumption. The nondeletion assumption is the elementary stipulation indicating the presence of an added assertion in a world. The world assumption is the elementary stipulation representing existence of a world. According to the invention, a method for testing assertions is provided for determining whether an assertion holds in a world. The method involves taking into account the presence of deletion nogoods relevant to the tested assertion. A deletion nogood is a nogood which indicates the contradiction between a world assumption and a nondeletion assumption which arises from a deletion of an assertion. Deletion nogoods are introduced at a world to block any further inheritance of an assertion from an ancestor world. The ATMS tool is further modified by replacing the traditional ATMS notion of inconsistency with a concept of inconsistency wherein only world assumptions are blamed for inconsistencies. The present invention may be used in connection with planning systems and diagnosis systems as well as with other types of knowledge-based systems.
Method for representing a directed acyclic graph of worlds using an assumption-based truth maintenance system