Q10 — RULE BASED EXPERT SYSTEMS

Seven engine-shop WMEs race through the Rete net; five tuples reach the conflict set and R1 out-ranks them all on both specificity and recency.

The Question

A Rete Net for classification of machines is shown in the figure. The labels A1, A2, A3, ..., A10, A11, A12, A13, ..., B1, B2, ..., B5, R1, R2, ..., R8, uniquely identify the nodes in the network. When required, use the given label ordering to break ties and to enter your answers.

Run the Rete algorithm for the Working Memory shown below, where the WMEs are in timestamp order. Assume that WMEs reside at appropriate Alpha nodes, and the Beta nodes point to WMEs residing in Alpha nodes.

101. (Fuel  ^item machine5 ^type diesel)
102. (Fuel  ^item machine2 ^type petrol)
103. (Fuel  ^item machine3)
104. (Power ^item machine4 ^lines 1)
105. (BoM   ^item machine4 ^part armature)
106. (BoM   ^item machine3 ^part armature)
107. (BoM   ^item machine5 ^part crankshaft)

For each WME identify its location (node label) in the Rete Net, and prepare the conflict set for the first cycle, then answer the given subquestions.

#	Question	Official answer
10.1	Which of the following rule-data tuples are in the conflict-set?	A, B, C, D, E
10.2	If the Inference Engine uses Specificity as the conflict resolution strategy then identify the rule-data tuples that will be ready to fire. If multiple rule-...	`R1,101,107` / `R1,107,101`
10.3	If the Inference Engine uses Recency as the conflict resolution strategy then identify the rule-data tuples that will be ready to fire. If multiple rule-data...	`R1,101,107` / `R1,107,101`

The Topic in 60 Seconds

Rete runs every WME through two halves:

Half	Nodes	Job
Alpha net	A1…A13	single-WME constant tests; each node stores every passing WME
Beta net	B1…B5	joins two token streams on shared variables ( $\text{item}=\langle x\rangle$ )

A production fires a rule-data tuple $(R, w_1, \dots)$ when tokens cover all its conditions. Live tuples = conflict set.

Deep-dive: browse the Concepts section for Rule-Based Expert Systems / Rete.

Step 0 — File each WME into its alpha node

WME	Path through the net	Binding
101 Fuel, diesel, machine5	A1 → A4 ( $x$ =m5) → A7	$x$ = machine5
102 Fuel, petrol, machine2	A1 → A4 ( $x$ =m2) → A9	$x$ = machine2
103 Fuel, no type, machine3	A1 → A4 only (A7/A9 reject — missing ^type)	$x$ = machine3, goes nowhere
104 Power, lines 1, machine4	A3 → A6 ( $x$ =m4) → A11	$x$ = machine4
105 BoM, armature, machine4	A2 → A5 ( $x$ =m4) → A12	$x$ = machine4
106 BoM, armature, machine3	A2 → A5 ( $x$ =m3) → A12	$x$ = machine3
107 BoM, crankshaft, machine5	A2 → A5 ( $x$ =m5) → A8	$x$ = machine5

Empty after all insertions: A10 (turbine-blades), A13 (lines 3) — so R3's join partner exists but mismatches, while R4, R5, R8 can never activate at all this cycle.

Step 1 — Beta joins and rule activations

Beta / Rule	Join tested	Result
B1 (A7 ⋈ A8 on $x$ )	A7: $\{m5\}$ · A8: $\{m5\}$	match at m5 → (R1, 101, 107)
R2 (A8 alone)	single condition	(R2, 107)
B2 (A9 ⋈ A8 on $x$ )	A9: $\{m2\}$ · A8: $\{m5\}$	no overlap → nothing
B3 (A9 ⋈ A10)	A10 empty	nothing
R5 (A10 alone)	empty	nothing
B4 (A11 ⋈ A12 on $x$ )	A11: $\{m4\}$ · A12: $\{m4, m3\}$	match at m4 → (R6, 104, 105)
R7 (A12 alone)	single condition	(R7, 105), (R7, 106)
B5 (A13 ⋈ A12)	A13 empty	nothing

10.1 — Conflict set → A, B, C, D, E

$\{(R1,101,107),\ (R2,107),\ (R6,104,105),\ (R7,105),\ (R7,106)\}$

Option	Tuple	Verdict
A	R1,101,107	Yes — diesel m5 meets crankshaft m5 in B1
B	R2,107	Yes — 107 sits in A8
C	R6,104,105	Yes — lines-1 m4 pairs with armature m4
D	R7,105	Yes — 105 passes A12
E	R7,106	Yes — 106 also passes A12
F	R3,102,107	No — B2 fails: petrol is m2, crankshaft is m5
G	R7,105,106	No — R7 is single-condition; it never takes two WMEs
H	R8,105,106	No — B5 needs lines-3, which no WME supplies

10.2 — Specificity → `R1,101,107`

Count each rule's WME patterns (conditions); most specific wins:

Tuple	Conditions counted	Specificity
R1,101,107	Fuel + diesel + BoM + crankshaft	4
R6,104,105	Power + lines-1 + BoM + armature	4 (tie)
R2,107	BoM + crankshaft	2
R7,105 / R7,106	BoM + armature	2

Tie between R1 and R6 at 4 conditions → question says break ties by label ordering → R1 < R6, so R1,101,107 fires.

10.3 — Recency → `R1,101,107`

Compare sorted-descending timestamp lists of each tuple's WMEs:

Tuple	Timestamps	Sorted desc
R2,107	107	[107]
R1,101,107	101, 107	[107, 101] ← wins
R7,106	106	[106]
R6,104,105	104, 105	[105, 104]
R7,105	105	[105]

Both R1's and R2's newest element is 107; the next comparison favours the tuple with additional (older) supporting WMEs: $[107,101] \succ [107]$ → R1,101,107.

Interactive trace — WMEs flowing through the net

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Working memory starts empty. No alpha node holds anything.

alpha nodejust activatedholding tokensrule in conflict set

Tricks & Tips

Answers: 10.1 A, B, C, D, E · 10.2 R1,101,107 / R1,107,101 · 10.3 R1,101,107 / R1,107,101