4. Constraint Satisfaction Problem Consider the problem of solving a 2x2 Sudoku puzzle. It is played on a grid consisting of 4 x 4 cells subdivided into four 2 x 2 squares as shown in the figure below. Some of the cells are empty, some of them are filled with digits from 1 through 4. The aim is to fill in the empty cells such that every row, every column, and every 2 x 2 square contains the digits 1, 2, 3, 4. Well- formed Sudoku puzzles do not require any guessing of cell assignments for solving them. 2 3 3 2 Formulate the general problem of solving 2 x 2 Sudoku as a constraint satisfaction problem. Describe the structure of the constraint graph associated with the constraint satisfaction problem. a)