Fill every empty cell with a hollow square or a filled one, so that each row and each column holds as many of one as of the other.

Never three alike side by side — not in a row, not in a column.

No two rows read the same, and no two columns do either.

Tap a cell for a filled square, tap again for a hollow one, once more to clear it. The squares the board came with stay put: tap one and they all give a shake, to show which they are.

Shown in red: three of the same kind side by side, squares a row or column has too many of, and two lines that read the same.

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