Basic Plan Representation

Cassandra's plan representation is an extension of that used in UCPOP [Penberthy and Weld 1992] and SNLP [McAllester and Rosenblitt 1991, Barrett et al. 1991], which is in turn derived from the representation used in NONLIN [Tate 1977]. A plan is represented as a schema with the following components:

• A set of steps;
• A set of anticipated effects of those steps;
• A set of links relating effects to the steps that produce and consume them (a step consumes an effect when it requires that effect to achieve one of its preconditions). Note that links in effect denote protection intervals, i.e., intervals over which particular conditions must remain true in order for the plan to work properly.
• A set of variable bindings instantiating the operator schema;
• A partial ordering on the steps;
• A set of open conditions, i.e., unestablished goals;
• A set of unsafe links, i.e., links the conditions of which could be falsified by other effects in the plan.