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Keen <p> Fill in the grid with digits from 1 to the grid size, so that every digit appears exactly once in each row and column, and so that all the arithmetic clues are satisfied (i.e. the clue number in each thick box should be possible to construct from the digits in the box using the specified arithmetic operation). <p> To place a number, click in a square to select it, then type the number on the keyboard. To erase a number, click to select a square and then press Backspace. <p> Right-click in a square and then type a number to add or remove the number as a pencil mark, indicating numbers that you think <em>might</em> go in that square.