Hierarchical inclusion

Figure 1.20: In a hierarchical model, the environment of one machine may itself contain a machine.

Under hierarchical inclusion, a plan is incorporated as an action in a larger plan. The original plan can be imagined as a subroutine in the larger plan. For this to succeed, it is important for the original plan to guarantee termination, so that the larger plan can execute more actions as needed. Hierarchical inclusion can be performed any number of times, resulting in a rooted tree of plans. This leads to a general model of hierarchical planning. Each vertex in the tree is a plan. The root vertex represents the master plan. The children of any vertex are plans that are incorporated as actions in the plan of the vertex. There is no limit to the tree depth or number of children per vertex. In hierarchical planning, the line between machine and environment is drawn in multiple places. For example, the environment, $E_1$, with respect to a machine, $M_1$, might actually include another machine, $M_2$, that interacts with its environment, $E_2$, as depicted in Figure 1.20. Examples of hierarchical planning appear in Sections 7.3.2 and 12.5.1.