24.9 Plan Recognition 537
reasons on goal achievement, including timing constraints. XFRM [59], depending
on the time constraints, executes either default plans or new plans obtained by
transforming the default plans by means of heuristic rules.
The Task Control Architecture (TCA), developed by Simmons [478, 479],
organizes processing modules around a central controller that coordinates their
interactions. A goal is decomposed into a task tree with subgoals that are achieved
by the decentralized modules. All communication is supported by the centralized
control. TCA provides control mechanisms for task decomposition and takes into
account temporal constraints in task scheduling. TCA is based on plans (called task
trees) that provide a language for