- The general problem is to find a solution that satisfies a set of constraints.
- heuristics used not to estimate
*the distance to the goal*but to decide what node to expand nest. - Examples of this technique are design problem, labelling graphs, robot path planning and cryptarithmetic puzzles (see last year).

**Algorithm:**

- Propagate available constraints:
- Open all objects that must be assigned values in a complete solution.
- Repeat until inconsistency or all objects assigned valid valid values:
- select an object and strengthen as much as possible the set of constraints that apply to object.
- If set of constraints different from previous set then open all objects that share any of these constraints.
- remove selected object.

- If union of constraints discovered above defines a solution return solution.
- If union of constraints discovered above defines a contradiction return failure

- Make a
*guess*in order to proceed. Repeat until a solution is found or all possible solutions exhausted:- select an object with a no assigned value and try to strengthen its constraints.
- recursively invoke constraint satisfaction with the current set of constraints plus the selected strengthening constraint.

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dave@cs.cf.ac.uk