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Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

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iii) The common value of each (sign-free) variable is then identified. The<br />

value of the set of variables, thus obtained, for each transition is called<br />

consistent bindings.<br />

2. A transition is enabled in the backward direction, if all its output places <strong>and</strong><br />

all but one of its input places possess consistent bindings.<br />

Father<br />

P1<br />

(X, Y)<br />

tr1<br />

An arc function<br />

¬(Y,X)<br />

¬(Y, X)<br />

Son<br />

Daughter<br />

Female<br />

(Y,X)<br />

¬(X,Y)<br />

An arc function at the output (input) arc of a transition is negative (positive).<br />

Fig. 8.13: A Petri net representing FOL clauses.<br />

An enabled transition fires by generating tokens following the arc function<br />

but with opposite sign of the arc functions <strong>and</strong> saves it at appropriate places,<br />

associated with the transition. However, a multiple copy of the same token is<br />

not kept at a given place.<br />

With reference to fig. 8.14, we find for transition tr1 two set of bindings<br />

X = d , Y = r (see place p1 <strong>and</strong> the associated arc function)<br />

<strong>and</strong> ¬Y = ¬ r , ¬X = ¬ d (see place p 2 <strong>and</strong> the associated arc function).<br />

The resulting consistent binding for tr1 is thus X = d & Y= r .<br />

P2<br />

P3<br />

P5<br />

(X)<br />

A place<br />

A transition<br />

tr2<br />

Mother<br />

P4

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