Resolving Open Conditions

The planning process is driven by the need to satisfy open conditions, which are initially simply the input goals. In the course of planning to satisfy an open condition, new subgoals may be generated; these are then added to the set of open conditions. The planner can establish an open condition in one of two ways: by introducing a new step into the plan, or by reusing an existing step by making use of one of its effects (see Figure 4). The secondary preconditions of the effect that establishes the condition become open conditions. If a new step is added, the preconditions of the step become open conditions as well. Finally, each time an open condition is established, a link is added to the plan to protect the newly established condition.

New step

Add to the plan a new step that has an effect that will establish the open condition. Add the step preconditions and the secondary preconditions of the effect as open conditions. The open condition becomes a completed link.

Reuse step

Make the open condition into a complete link from an effect of an existing plan step. Add the secondary preconditions of the effect as open conditions.

One way of establishing a condition is simply to notice that the condition is true in the initial state. Because the initial conditions are treated as the results of the start operator, which is always a part of the plan, this method can be treated as establishment by reusing an existing step. Indeed, this simplification is the motivation for representing the initial conditions in this way.