Resolving Threats to Unsafe Links

Figure 14 shows how threats are resolved. The methods shown in steps 2, 3, and 4 are standard methods found in SNLP and UCPOP; they are often termed separation, demotion, and promotion respectively. We say that the methods in step 5 disable the threat. The methods in steps 5a and 5b ensure that the threatening effect does not occur in a given contingency. The method in step 5a is a modification of a standard method found in UCPOP and other planners that use secondary preconditions. Essentially, the idea is to prevent an effect from occurring by ensuring that the context in which it occurs cannot hold. The method in 5b prevents an effect from occurring in a contingency by forbidding the execution of the step that produces it. The method in step 5c notes that the established step or effect cannot occur in a given contingency. If any of these techniques result in inconsistent labeling of any plan element (so that, for example, it cannot occur in every contingency in which it is required) the resulting partial plan is abandoned, as it represents a dead end in the search space.

Resolve(Plan, Unsafe)

1. Initialize a list NewPlans;
2. If the unification of the clobbering condition ClobberCond with the condition Cond established by the link Unsafe involves adding codesignation constraints to the bindings of Plan:
   • Make each possible modification to the bindings of Plan that ensures that ClobberCond cannot unify with Cond;
   • Add each resulting partial plan to NewPlans;
3. If the clobbering step ClobberStep can precede the step Step that establishes Unsafe:
   • Add an ordering to ensure that ClobberStep precedes Step;
   • Add the resulting partial plan to NewPlans;
4. If the step SupStep supported by Unsafe can precede ClobberStep:
   • Add an ordering to ensure that SupStep precedes ClobberStep;
   • Add the resulting partial plan to NewPlans;
5. Prevent the clobbering effect ClobberEff from occurring in each contingency Conting in which the link Unsafe is unsafe:
   • Do one of:
     1. Add the negation of the secondary preconditions of ClobberEff as an open condition with positive contingency label Conting;
     2. Add Conting to the negative contingency labels of ClobberStep;
     3. Add Conting to the negative contingency labels of the effect SupEff or step SupStep that Unsafe supports;
   • If appropriate modify the relevant decision-rule as discussed in Section 4.2.4;
   • Add orderings to ensure that step ClobberStep occurs between steps Step and SupStep;
   • Propagate labels as appropriate;
   • Add each resulting partial plan to NewPlans;
6. Return NewPlans.