Links and Open Conditions

A link represents a causal dependency in the plan, specifying how a condition Cond is established by an effect Eff, which has Cond as a postcondition. Eff has secondary preconditions SecPre and is a result of step Step. The link supports the step SupStep or effect SupEff through the condition Cond which is one of:

• An enabling precondition of SupStep;
• A secondary precondition of an effect SupEff that is a result of SupStep;
• The negation of a secondary precondition of an effect that is a result of SupStep, thus preserving a link.

A link is unsafe in a contingency Conting in which it is required if there is a clobbering effect ClobberEff with postcondition ClobberCond (the clobbering condition) resulting from step ClobberStep such that:

• Either ClobberCond can unify with Cond;
  Or Cond is of the form (know-if KnowCond), and ClobberCond can unify with KnowCond;
• Step ClobberStep can occur between steps Step and SupStep;
• Effect ClobberEff can occur in contingency Conting.

An open condition (an unachieved subgoal) is represented in Cassandra as an incomplete link, i.e., a link missing the information about the effect that establishes it.