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# <ConsistencyModule:> ?ExprX #= ?ExprY

ExprX is equal to ExprY.
*ExprX*
- Integer arithmetic expression
*ExprY*
- Integer arithmetic expression

## Description

Constrains ExprX and ExprY to be equal. Also constrains all variables
appearing in ExprX and ExprY to be domain variables and checks that all
constants and ground sub-expressions are integers.

ConsistencyModule is the optional module specification to give the
consistency level for the propagation for this constraint: gfd_bc
for bounds consistency, and gfd_gac for domain (generalised arc)
consistency. Note that some operators (integer division, modulus)
only support bounds consistency and will be propagated with bounds
consistency even when posted with gfd_gac.

This constraint is implemented using Gecode's MiniModel's rel() for
both integer and boolean expressions, with sub-expressions/constraints
not supported by MiniModel factored out and posted as auxiliary
constraints.

A more restricted version of this constraint is defined in the
global constraint catalog as 'eq'. Here the constraint is defined
between two domain variables rather than two expressions.

### Modules

This predicate is sensitive to its module context (tool predicate, see @/2).
## Examples

[eclipse 28]: A :: [1,3,5,7], A #= B.
A = A{[1, 3, 5, 7]}
B = B{[1 .. 7]}
[eclipse 29]: A :: [1,3,5,7], gfd_gac: (A #= B).
A = A{[1, 3, 5, 7]}
B = B{[1, 3, 5, 7]}
[eclipse 30]: A :: [1,3,5,7], gfd_gac: ( A #= B + 1).
A = A{[1, 3, 5, 7]}
B = B{[0, 2, 4, 6]}

## See Also

#< / 2, #=< / 2, #>= / 2, #> / 2, #\= / 2, #= / 3, suspend : #= / 2, ic : #= / 2, fd : #= / 2