This panel is aimed at the issue of how to use and modify plans during the course of execution. The relationship between a plan and the actions that an agent taJces has generated a great deal of interest in the past few years. This is, in part, a result of the realization that planning in the abstract is an intractable problem and that much of the complexity of behavior is best understood in terms of the complexity of the environment in which that behavior occurs. This panel presents five distinct personalities and approaches to this problem: • Agre looks at replacing "planning" with situated activity. In particular, he has been considering the problems involved with the reference assumptions of classical planning. • Firby's hierarchical planner has primitive actions that are instantiated at execution-time. The execution of these primitives generates information that can be used to guide selection of later operators. • In Alterman's model of run-time adaptation, the executive responds to failures by using external cues to move between alternative steps or approaches stored in an existing network of semantic/episodic information. • Simmons has been exploring techniques to create robust, reactive systems that can handle multiple tasks in spite of the robot's limited sensors and processors. His approach takes full advantage of the resources that the robot does have. This includes using hierarchical coarse-to-fine control strategies, using concurrency whenever feasible, and explicitly focusing attention on the robot's tasks and monitored conditions. • H a m m o n d suggests a theory of agency which casts planning as embedded within a memorybased understanding system connected to the environment. Within this approach, the environment, plan selections, decisions, conflicts and actions are viewed through the single eye of situation recognition and response.
