A set of constraints is created for the Sudoku puzzle. The ConstraintProgram class is responsible for providing these. Each variable has basically 4 constraints associated with it: Constraint 1: position/location: Each cell maybe taken up by ONLY 1 value. Constraint 2: row: Each number from 1 to 9 can only be present once in a row. Constraint 3: column: Each number from 1 to 9 can only be present once in a column. Constraint 4: region: Each region (a smaller grid of size 3x3) can contain a number from 1 to 9 at most once.