format sudoku

instances {
  line "# size={n:1..} box={box_width:1..}x{box_height:1..}"
  cells[n][n]: alphabet "123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ0" blank "."

  check box_width * box_height == n "boxes must tile the grid"
  check max(cells) <= n "a value is larger than the size of the grid"
}