Sudoku is a complex logic puzzle with simple rules. Fill in the 9x9 grid so that the digits 1 - 9 appear only once in each row, column and group (sometimes called box.) We've blanked out most of a puzzle to illustrate this below.

Irregular-group puzzles follow the same rules.

The X variants add 2 more constraints: both diagonals must also contain the digits 1-9.

That's it.

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