Normal sudoku rules apply. Each row, column, and 3x3 box contains two 'indexer' cells: a 'row indexer' and a 'column indexer'. The digit in a row indexer gives the position of the column indexer in that row. The digit in a column indexer gives the position of the row indexer in that column. Positions are counted left-to-right in rows and top-to-bottom in columns . Digits in a cage sum to the small clue in the upper left corner, if given. Indexer cells cannot appear in cages.