3.1 Finite Domain Constraints

Finite domain (FD) constraints have become quite popular and are widely supported. A finite domain variable denotes an integer. However, we may not know exactly which integer. Typically we only know that it must be one of a finite number of possibilities, e.g. one of 1,2 or 7. This would be represented by a basic constraint:

Suppose, we have a second FD variable . If we unify them , their domains are intersected thus resulting in the conclusion . This technique underlies the efficient treatment of agreement: if encodes the agreement information on one word and on another and the 2 words must agree, then the constraint enforces this agreement.