The notion of "cognizant failure" or the knowledge that someting has gone wrong has driven the development of this layer. The structure of the sequencing layer follows that of Firby in which a task queue is maintained. The task queue, which dictates the actions of the agent, consists of a list of methods for performing a task. A method consists of primitive tasks or methods which are iteratively decomposed into the primitive actions of the agent. Each task is tried and decomposed until either completion or failure. Upon failure, an alternative method is tried.
The sequencing layer specifically manages the initiation and termination of primitive action and the intiation and termination of the decomposition of methods. Subtle issues regarding the initiation and termination of primitive actions include the allocation and reclamation of resources and the management of resource conflicts through the use of semaphores.
Deliberation, from the perspective of the sequencing layer, is a primitive task which acts on the internal task queue and the internal world model rather than directly on the environment. Like external actions, deliberation is initiated and terminated by the sequencing layer.