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Instead of merely posting constraints it is often useful to reflect its truth value into a 0/1-variable B, so that:
This mechanism is known as reification. Several frequently used operations can be defined in terms of reified constraints. A reified constraint is written:
| ?- Constraint #<=> B.
where Constraint is reifiable. As an example of a constraint that
uses reification, consider
defined to be true if X occurs exactly N times in the list
L. It can be defined thus:
exactly(_, , 0). exactly(X, [Y|L], N) :- X #= Y #<=> B, N #= M+B, exactly(X, L, M).
Finally, reified constraints can be used as terms inside arithmetic expression. The value of the term is 1 if the constraint is true, and 0 otherwise. For example:
| ?- X #= 10, B #= (X#>=2) + (X#>=4) + (X#>=8). B = 3, X = 10