*NEEDED*

In Figure 1 at the RELATION-20 node, there are two tests. The first test (test 0) has the following DEP precondition:

rest-prop: check-con (C (sur (form prog)))

This indicates that the rest proposition, in this case DEATH-1, must have a constraint imposed on it of the form (C (sur (form prog))). This information is recorded in *NEEDED* as shown below, as well as the corresponding need for the DEP precondition in the second rule (rule 1).

*NEEDED*

RELATION-20 rule 0 precondition 0:
((%DEATH-1 (C (SUR (FORM PROGRESSIVE)))))

(RELATION-20 rule 1 precondition 0:
((%DEATH-1 (C (SUR (FORM NOUN)))))

The information in *NEEDED* will be used together with *CON* below to determine all of the rules that can supply the constraints for each precondition. The mirror of *NEEDED*, *NEED-FILLERS*, will also be created during optimization. *NEED-FILLERS* will, for each constraint, list the node(s) for which that constraint supplies a precondition. Whenever a rule is failed, then, each of the rule's effect constraints are checked in *NEED-FILLERS* to determine the other rules whose preconditions would have been met by the failed rule's effects. Each of these rules that are affected are then looked up in *NEEDED* to determine if there are any other rules that are still valid that could supply the condition. If not, then that rule is also failed. This is the central mechanism behind the soundness processing.