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A(X,Y):- X>0, B(X,Y).
Like in regular logic programming, evaluating a goal, such as <code>A(X,1)</code> requires evaluating the body of the last clause with <code>Y=1</code>. Like in regular logic programming, this in turn requires proving the goal <code>B(X,1)</code>. Contrary to regular logic programming, this also requires a constraint to be satisfied: <code>X>0</code>, the constraint in the body of the last clause.
Whether a constraint is satisfied cannot always be determined when the constraint is encountered. In this case, for example, the value of <code>X</code> is not determined when the last clause is evaluated. As a result, the constraint <code>X>0</code> is not satisfied nor violated at this point. Rather than proceeding in the evaluation of <code>B(X,1)</code> and then checking whether the resulting value of <code>X</code> is positive afterwards, the interpreter stores the constraint <code>X>0</code> and then proceeds in the evaluation of <code>B(X,1)</code>; this way, the interpreter can detect violation of the constraint <code>X>0</code> during the evaluation of <code>B(X,1)</code>, and backtrack immediately if this is the case, rather than waiting for the evaluation of <code>B(X,1)</code> to conclude.
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