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### Contingency Labels

Every step, effect and open condition in a partial plan has two sets of contingency labels attached to it. In the interests of brevity, we also refer to the labels of a link; in this case, we mean the labels of the step or effect that the link establishes.

Each contingency label has two parts: a symbol representing the source of uncertainty, and a symbol representing a possible outcome of that source of uncertainty. Positive contingency labels denote the circumstances in which a plan element must or will necessarily occur; negative contingency labels denote the circumstances in which a plan element cannot or must not occur.

Contingency labels must be propagated through the plan. In general, positive contingency labels are propagated from goals to the effects that establish them, while negative contingency labels are propagated from steps to the effects that result from them. The details are as follows:

• A step inherits the positive labels of the effects that result from it;
• A step inherits the negative labels of the effects that establish its enabling preconditions;
• An effect inherits the positive labels of the steps whose enabling preconditions it establishes;
• An effect inherits the positive labels of the effects whose secondary preconditions it establishes;
• An effect inherits the negative labels of the step from which it results;
• An effect inherits the negative labels of the effects that establish its secondary preconditions;
• An open condition inherits the positive labels of the step or effect that it is required to establish.
Cassandra's system of label propagation is based on that of CNLP but is more complex. Indeed, it is rather more complex than we would like. This complexity is mandated by the need to deal with operators that involve multiple context-dependent effects, which has the result that a step and its effects do not necessarily share the same labels.

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Louise Pryor <louisep@aisb.ed.ac.uk>;