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# ?B isd ?C

The constraint expression C evaluates to the boolean value B.
*?B*
- A variable with domain 0..1.
*?C*
- An arithmetic constraint expression.

## Description

This is an evaluation constraint. It states that the constraint
expression C evaluates to the boolean value B, where the value 0 means
false and 1 true. This constraint can be used both to test the validity
of the constraint expression (entailment test) and to impose this
constraint or its negation. For the former, B will be instantiated as
soon as either the constraint expression or its negation is subsumed by
the current state of its domain variables. For the latter, when B is
instantiated, then depending on its value, either the constraint
expression or its negation will be imposed on its arguments.

### Fail Conditions

Fails if both C is true.
### Resatisfiable

No.
## See Also

#=> / 3, #<=> / 3, #\/ / 3, #/\ / 3, is / 2