Among rules of the same salience, newly activated rules are placed using the OPS5 strategy of the same name. First the recency of the pattern entities that activated the rule is used to determine where to place the activation. Every fact and instance is marked internally with a ìtime tagî to indicate its relative recency with respect to every other fact and instance in the system. The pattern entities associated with each rule activation are sorted in descending order for determining placement. An activation with a more recent pattern entities is placed before activations with less recent pattern entities. To determine the placement order of two activations, compare the sorted time tags of the two activations one by one starting with the largest time tags. The comparison should continue until one activationís time tag is greater than the other activationís corresponding time tag. The activation with the greater time tag is placed before the other activation on the agenda.
If one activation has more pattern entities than the other activation and the compared time tags are all identical, then the activation with more time tags is placed before the other activation on the agenda. If two activations have the exact same recency, the activation with the higher specificity is placed above the activation with the lower specificity. Unlike OPS5, the not conditional elements in CLIPS have pseudo time tags which are used by the LEX conflict resolution strategy. The time tag of a not CE is always less than the time tag of a pattern entity, but greater than the time tag of a not CE that was instantiated after the not CE in question.
As an example, the following six activations have been listed in their LEX ordering (where the comma at the end of the activation indicates the presence of a not CE). Note that a factís time tag is not necessarily the same as itís index (since instances are also assigned time tags), but if one factís index is greater than another factsís index, then itís time tag is also greater. For this example, assume that the time tags and indices are the same.
rule-6: f-1,f-4 rule-5: f-1,f-2,f-3, rule-1: f-1,f-2,f-3 rule-2: f-3,f-1 rule-4: f-1,f-2, rule-3: f-2,f-1
Shown following are the same activations with the fact indices sorted as they would be by the LEX strategy for comparison.
rule-6: f-4,f-1 rule-5: f-3,f-2,f-1, rule-1: f-3,f-2,f-1 rule-2: f-3,f-1 rule-4: f-2,f-1, rule-3: f-2,f-1