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Range-Restricted Rules |
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Range-Restricted Rules |
OntoBroker implements negation as the so-called "default negation" (OntoBroker supports both stratified and wellfounded negation). This means that when you have the program:
q(a).
q(b).
r(a).
QUERY q1: ?- q(?X) AND NOT r(?X).
Then OntoBroker evaluates the rule as follows:
| 1. | Retrieve all tuples for q(X): X=a, X=b |
| 2. | Retrieve all tuples for r(X): X=a |
| 3. | Subtract the tuples r(X) from q(X). |
So the result of the query is "X=b" because b does not occur in r(X) and hence the formula "NOT r(X)" is true.
Imagine what would happen if we had the program:
q(a).
q(b).
r(a).
QUERY q1: ?- NOT r(?X).
We have nothing as the base where we could subtract the r(X) facts from. So the result would be infinitely large (= the result size is not range restricted).