COMP 4200 Expert Systems Dr Christel Kemke Department

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

Rule-based Systems Control Procedural Control Conflict Resolution Strategies Salience Modules © C. Kemke Control 2

Procedural Control if-then-else while-do © C. Kemke Control 3

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

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 )). . . ) © C. Kemke Control 5

Rule Activation and Execution © C. Kemke Pattern Matching, Rule Activation, Rule Execution Conflict Set, Conflict Resolution, Strategies Control 6

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. © C. Kemke Control 7

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. © C. Kemke Control 8

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 firstout). There are different types of Conflict Resolution Strategies. © C. Kemke Control 9

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. © C. Kemke Control 10

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

Conflict Resolution: 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) © C. Kemke Control 12

Conflict Resolution: Recency © C. Kemke 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) Control 13

Explicit Control © C. Kemke Salience Meta-Rules Control 14

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. © C. Kemke Control 15

Rule Prioritization in Clips © C. Kemke for example, consider the following rules. . . (forced order of execution) Control 16

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) ) © C. Kemke Control 17

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

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. © C. Kemke Control 19

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)) © C. Kemke Control 20

Reasoning Control Classes of Rules © C. Kemke Control 21

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. © C. Kemke Control 22

Categories Example (deffacts control-information (phase detection) (phase-after detection isolation) (phase-after isolation recovery) ) © C. Kemke Control 23

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) ) © C. Kemke Control 24

Another Way. . . (deffacts control-information (phase detection) (phase-sequence isolation recovery detection)) © C. Kemke Control 25

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)) ) © C. Kemke Control 26

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. © C. Kemke Control 27

Explanation © C. Kemke 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) Control 28

Control: Meta-Rules © C. Kemke 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. Control 29