Sudoku

The case: A 9×9 grid of cells. Some cells already hold a suspect's number; others are empty.

The rules: Place the digits 1 through 9 such that every row, every column, and every 3×3 precinct contains each digit exactly once.

The job: Click an empty cell, then click a number to place it. Click a placed number to remove it. The case closes when every cell is filled and no rule is broken.