Protecting Unsafe Links

Whenever an open condition is established, links in the plan may be jeopardized either because a new step threatens an existing link, or because a new link is threatened by an existing step. The situations in which a link is unsafe are shown in Figure 5. In general, a link is considered unsafe if there is an effect in the plan that could possibly interfere with the condition established by that link.

There are three general methods of protecting a threatened link (see Figure 5). First, ordering can be used to constrain the threatening action to occur either before the beginning or after the end of the threatened link. Second, the threatening effect and the threatened link can be separated by imposing constraints on the variables involved so that the effect cannot be unified with the established condition. Third, the link can be preserved by generating a new subgoal to disable the effect that threatens the link.

A link establishing the condition Cond is unsafe if there is an effect Eff in the plan (other than the effect SourceEff that establishes Cond and the (possible) effect GoalEff that is either established or disabled by the link) with the following properties:

Unification

One of the postconditions in Eff can possibly unify with either Cond or its negation.

Ordering

The step that produces Eff can, according to the partial order, occur both before the step that produces GoalEff and after the step that produces SourceEff.

Ordering

Modify the ordering of the steps in the plan to ensure that the step producing Eff occurs either before the step that produces SourceEff or after the step that produces GoalEff;

Separation

Modify the variable bindings of the plan to ensure that the threatening effect Eff cannot in fact unify with the threatened condition Cond;

Preservation

Introduce a new open condition in the plan to disable Eff. This new open condition is the negation of one of Eff's secondary preconditions.