##### 10.34.2.3 Reified Constraints

Instead of merely posting constraints it is often useful to reflect its truth value into a 0/1-variable B, so that:

• the constraint is posted if B is set to 1
• the negation of the constraint is posted if B is set to 0
• B is set to 1 if the constraint becomes entailed
• B is set to 0 if the constraint becomes disentailed

This mechanism is known as reification. Several frequently used operations can be defined in terms of reified constraints, such as blocking implication [Saraswat 90] and the cardinality operator [Van Hentenryck & Deville 91], to name a few. A reified constraint is written:

```     | ?- Constraint #<=> B.
```

where Constraint is reifiable. As an example of a constraint that uses reification, consider `exactly(`X`,`L`,`N`)`, which is 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).
```

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