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An agent's hierarchy maintenance function can be employed to help maintain consistency, illustrated notionally in Figure 2. The agent architecture identifies assertions at each of the higher levels in the hierarchy that led to a new subtask. These assertions together form a subtask "support set." In Figure 2, assertions a14, a15, e14, and e19 **form the support**
set for Subtask2 while a12, a22, e22 "support" Subtask3**. These support sets, in effect, form**
justifications for subtasks in the hierarchy. When an assertion in a support set is removed
(e.g., a22), the agent responds by removing the subtask (Subtask3**). While not all hierarchical architectures use architectural processes to create and destroy subtask databases, this**
example illustrates how an architectural hierarchical maintenance function can be realized via a process similar to that of justification in truth maintenance.

Within a specific subtask, reason maintenance can go on as before. However, the hierarchical structure adds a complication to the maintenance of logical consistency. Assumptions at some level in the hierarchy can be dependent on entailments and assumptions in higher levels of the hierarchy.1 This dependence relationship is suggested in Figure 1 by the **curved**
lines extending from one subtask to the one below it. Higher levels of the hierarchy form a "context" for reasoning in the local subtask.

For execution agents embedded in dynamic domains, the hierarchical context may change at almost any time. The changing context is not problematic for entailments; the