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Formally, constraint logic programs are like regular logic programs, but the body of clauses can contain constraints, in addition to the regular logic programming literals. As an example, <code>X>0</code> is a constraint, and is included in the last clause of the following constraint logic program.
B(X,1):- X<0.
B(X,Y):- X=1, Y>0.
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.
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