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:
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:
(know-if KnowCond), and ClobberCond can unify with KnowCond;
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.