A number game (Sudoku) is a puzzle in which missing numbers are to be filled into a 9 by 9 grid of squares
which are subdivided into 3 by 3 boxes so that every row, every column, and every box contains the numbers 1 through 9.
A CLASSIC Sudoku is a symmetrical Sudoku that has one and ONLY one solution.
A symmetrical Sudoku is a Sudoku in which any two symmetrical squares (around the center square) are either
both filled with numbers or both empty.