COMP 4200: Expert Systems

1. COMP 4200: Expert Systems Dr. Christel Kemke Department of Computer Science University of Manitoba

2. Rule-based Systems Control • Procedural Control • Conflict Resolution Strategies • Salience • Modules

3. Procedural Control if-then-else while-do

4. Procedural Control in Actions • Procedural Control Elements can appear on the RHS of a rule or in message-handlers of classes. (if <predicate-expression> then <expression>+ [else <expression>+ ]) ;;else-part optional (while <predicate-expression> [do]<expression>* ]) ;;‘do’ not mandatory

5. Example – if-then-else (defrule special-age “18, 21, 100” (or (person (name ?name) (age ?age&18)) (person (name ?name) (age ?age&21)) (person (name ?name) (age ?age&100))) => (if (= ?age 18) then (printout t ?name “ can buy beer in Canada.”) else (if (= ?age 21) then (printout t ?name “ can buy beer in the USA.”) else (if (= ?age 100) then (printout t “The major will visit ” ?name ))...)

6. Rule Activation and Execution Pattern Matching, Rule Activation, Rule Execution Conflict Set, Conflict Resolution, Strategies

7. Facts, Rules, Pattern Matching • Forward-chaining systems begin with an initial set of facts and in an inference process generate new facts until a goal state is reached. • Facts are entered into the Working Memory (WM). • The conditions of each rule are matched to these facts  Pattern Matching (e.g. Rete Algorithm) • Rules whose conditions are satisfied are activated and entered on the agenda.

8. Rule Activation • The pattern matching algorithm determines, in which sequence rules are being activated, i.e. placed on the agenda. • In CLIPS, you cannot easily determine the sequence of rule activations. • Thus, do not write programs, which implicitly depend on a certain sequence of facts activating rules.

9. Conflict Resolution • The set of activated rules on the agenda is called the Conflict Set. • Choosing which of the activated rules to fire next is known as Conflict Resolution. • A simple strategy is to select rules according to the order, in which they are put on the agenda. CLIPS, for example, uses a stack (last-in first-out). • There are different types ofConflict Resolution Strategies.

10. Conflict Resolution Strategies There are two categories of strategies: • global strategies • local strategies Global Strategies are domain-independent and part of the Rule Interpreter/Inference Engine. Local Strategies are domain-dependent and implemented as part of the Rule Base.

11. Conflict Resolution: Refractoriness • Refractoriness Forward chainers typically implement a refractory conflict resolution strategy - once a rule is fired, it isn't used again on the same data

12. Conflict Resolution: Specificity • Specificity: Choose a rule with the most conditions or the most specific conditions ahead of a more general rule (prefer most specific knowledge instead of general knowledge)

13. Conflict Resolution: Recency • Recency: Fire a rule first that is activated by a fact just added to Working Memory, i.e. fire most recently activated rule first ( focus on one line of reasoning, with all related facts and rules)

14. Explicit Control Salience Meta-Rules

15. Salience • We can use salience measures to prioritize rules. • CLIPS provides a built-in method for prioritizing rules: (declare (salience value)) • Salience values can range from -10000 to +10000. Default is 0. • We can thus force the execution of one rule over another. We can implement sequencing of rules.

16. Rule Prioritization in Clips • for example, consider the following rules... (forced order of execution)

17. Two Nifty Rules (defrule fire-first (declare (salience 30)) (priority first) => (printout t "Print First" crlf) ) (defrule fire-second (declare (salience 20)) (priority second) => (printout t "Print Second" crlf) )

18. And One More... (defrule fire-third (declare (salience 10)) (priority third)=> (printout t "Print Third" crlf) )

19. Getting Ready to Run... (assert (priority second)) (assert (priority first)) (assert (priority third)) (agenda) 30 fire-first: f-2 20 fire-second: f-1 10 fire-third: f-3 For a total of 3 activations.

20. Running This... • The CLIPS agenda acts like a stack - last rule on, first fired • If salience were not used, the third rule, then the first, then the second would fire, due to the sequence of facts and activation of rules: (assert (priority second)) (assert (priority first)) (assert (priority third))

21. Reasoning Control Classes of Rules

22. Categories of Rules • Salience values are arbitrary; often what we want is that a certain class of rules are considered before others. • This can be built into the rules themselves using a kind of 'tag' IF (status is check-for-emergencies) .... and employed by setting a fact to allow various categories of rules to be selected - for example, asserting that the status is check-for-emergencies • Another rule can be implemented to change status to the next group of rules.

23. Categories Example (deffacts control-information (phase detection) (phase-after detection isolation) (phase-after isolation recovery) )

24. Categories Example • Context Limiting (defrule change-phase (declare (salience -10)) ?phase <- (phase ?current-phase) (phase-after ?current-phase ?next-phase) => (retract ?phase) (assert (phase ?next-phase) )

25. Another Way... (deffacts control-information (phase detection) (phase-sequence isolation recovery detection))

26. Another Way... (defrule change-phase (declare (salience -10)) ?phase <- (phase ?current-phase) ?list <- (phase-sequence ?next-phase $?other-phases) => (retract ?phase ?list) (assert (phase ?next-phase) (assert (phase-sequence ?other-phases ?next-phase)) )

27. Explanation • Note the $?other-phases • The $ operator causes the variable to be bound to several symbols (the remainder of the list of phases, in this case) rather than one. Note that the $ is not part of the variable name and isn't used later when referencing the variable.

28. Explanation • This approach could be used in a monitoring or control system - forward reasoning is typically used in such systems because we get information, then want to see the ramifications of it (analyze, provide treatment, and so on)

29. Control: Meta-Rules • Meta-Rules Use Meta-Rules to divide rules into classes. Choose one class over another at a given point. This implements domain-dependent knowledge about which set of rules to use during reasoning. CLIPS provides a Module-construct with similar effects.