The goal of the game is to fill in a grid with 81 digits (size of 9x9 cells)
with remaining digits 1 up to 9 so that no digit appears in a row, a column, or
a box (size of 3x3 cells) no more than once. The difficulty of the Sudoku game
is particularly given how difficult is to find out what digit to fill and the
total number of cells that are empty. You can write hints i.e. small digits you
think that are candidate digits in a given cell. You can use candidate digits
not only for overview but also for simple or advanced strategies.
