Implementing an Incremental Hierarchical Plan Recognition System

THE TASK OF INCREMENTAL PLAN RECOGNITION CAN BE COMPUTATIONALLY EXPEN- SIVE, EVEN IN A SIMPLE DOMAIN. DERIVING ALL POSSIBLE MEANINGS OF A PARTI- CULAR ACTION, GIVEN THE LIMITED CONTEXT OF PREVIOUS ACTIONS, CAN LEAD TO AN EXPLOSION OF COMPETING INTERPRETATIONS, MANY MORE THAN EXPECTED AT FIRST GLANCE. THERE ARE AT LEAST TWO CAUSES. SOME INTERPRETATIONS HAVE TO BE CONSIDERED VIABLE BECAUSE UNBOUND PARAMETERS ARE POSSIBLY CONSISTENT, AND MULTIPLE TOP-LEVEL GOALS CAN BE IN PROGRESS SIMULTANEOUSLY. THIS PAPER EXAMINES A SET OF TECHNIQUES THAT CAN BE USED TO CONTAIN THE PROLIFERATION OF ALTERNATIVE PLAN INTERPRETATIONS AS SOON AS POSSIBLE DURING THE RECOGNI- TION PROCESS. THESE IDEAS ARE IMPLEMENTED IN A SYSTEM CALLED `GRAPPLE''. GRAPPLE IS A HIERARCHICAL PLAN RECOGNITION SYSTEM USED IN AN INTELLIGENT INTERFACE FOR RECOGNIZING USER GOALS FROM LOW-LEVEL ACTIONS.