chapter 3

Consistency-Enforcing and Constraint Propagation

Knowledge of what is possible is the beginning of happiness.

George Santayana, Little Essays

Perhaps the most exciting and fundamental concept that drives the constraint processing area is constraint propagation. These are inference methods used by us in everyday life that can be imitated by computers to exhibit intelligent inference.

Assume again our party example in Chapter 2 where we would like to invite Alex, Bill, and Chris to a party. Let A, B, and C denote the propositions “Alex comes,” “Bill comes,” and “Chris comes,” respectively. If Alex comes to the party, Bill will come as well, and if Chris comes, then Alex will too. This is expressed using Boolean constraints as (A

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